On April 30, 1942, Ronald W. Harker, a test pilot for the British Rolls-Royce engine manufacturer, took a brief hop in a RAF Mustang at the airbase at Duxford. Like lots of other pilots, he was highly impressed with the Mustang. It was 30 mph faster than the Spitfire VB at similar power settings and had nearly twice the range. Upon landing, he is reported to have said that the airplane would be a natural for the new Merlin 60 series of engines that Rolls Royce was just beginning to produce. The Merlin 60 had originally been intended for the pressurized high-altitude Wellington VI bomber, but had hastily been adapted to the Spitfire VIII.
Rolls Royce management was intrigued by the idea and immediately jumped into action. They requested that three Mustangs be loaned to them so that they could fit them with Merlins. Rolls Royce studied various Merlins, including the single-stage Mk XX and the two-stage Mk 61. The two-stage Merlin was the better choice because of its far superior high-altitude performance. The Merlin Mk 61 engine crankshaft was geared to two supercharger blowers stacked in series. Because of the rapid compression of air, the temperature of the air after it passed through both stages of the supercharger increased by 200 centigrade degrees. In order to lower this temperature and thus increase the mass flow of the air entering the engine, an intercooler was added, requiring an extra radiator. After much thought, it was decided to mount the extra radiator underneath the nose, in the same duct as the ram inlet for the updraft carburetor.
This conversion was authorized on August 12, 1942. Initially, three Mustang Is were allocated to the program, but two more were added later. Their RAF serials were AM121, AM208, AL975, AM203, and AL963. They were assigned the designation Mustang X. No two of these Mustang Xs were exactly alike, but they all featured small chin-type radiators mounted underneath the engine, all had four-bladed propellers to absorb the extra engine power, and they were all powered by the Merlin 65, which in comparison with the Merlin 66 had a lower full-throttle height but gave higher power at lower altitudes. In comparison to the Allison V-1710, it was 205 hp more powerful at 20,000 feet and 490 hp more powerful at 25,000 feet. The first Mustang X (AL975) took to the air on October 12, 1942, piloted by Captain R. T. Shepherd. It initially had a regular Spitfire IX Rotol propeller but was later fitted with a lerger specially-designed propeller. AL963 flew for the first time on November 13, 1942, and AM121 followed on December 13. AM121 went to the AFDU at Duxford for service evaluation. The fourth and fifth were evaluated by the USAAF in full USAAF markings. These Mustang Xs were to be kept busy throughout the rest of the war, testing various later marks of the Merlin engine.
The performance of these re-engined aircraft was excellent, with maximum speed obtained at Boscombe Down being 433 mph at 22,000 feet. However, the yaw stability was degraded by the increased side area of the nose. The success of these tests led Rolls Royce to propose the production of 500 Merlin 65 engines to re-engine most of the RAF's Mustang fleet to Mark X standards. However, there was no place where these conversions could be done, and such plans were never carried out.
One of the more bizarre proposals considered by Rolls-Royce was the possible installation of a 2400 hp Rolls-Royce Griffon 63 engine mounted amidships in a Mustang airframe a la P-39 Airacobra, driving a contrarotating propeller via an extension shaft. The cockpit was to be moved forward to a position well ahead of the wing. It was anticipated that this modification would make it possible to achieve speeds as high as 500 mph. A mockup of this configuration was carefully prepared, but the concept was abandoned before work could proceed any farther.
Meanwhile, in May of 1942, Rolls-Royce had informed Major Thomas Hitchcock, US military attache in London, that they planned to convert Mustang airframes to the Merlin engine. It just so happened that Major Hitchcock had been thinking of just this idea himself. He passed the word along to Wright Field and to North American Aviation. The idea attracted immediate interest. It just so happened that at this very time negotiations were taking place with the Packard Motor Car Company of Detroit, Michigan for license manufacture in the United States of the new Merlin engine with the two-stage supercharger. On July 25, 1942, North American was authorized to convert two Mustangs to Merlin 65 engines imported from England. These aircraft were considered sufficiently different from the existing Mustang that they were given a new designation--XP-78.
NAA selected two P-51s from the batch of Mustang IAs that had been repossessed from the RAF by the USAAF. Their serial numbers were 41-37350 and 41-37421. NAA gave the project the company designation NA-101. The designation of these two aircraft was changed to XP-51B while the work was progressing. Although the early work by Rolls-Royce in conversion of Mustangs to the Merlin engine provided valuable insight to North American engineers, the British engine manufacturer did not directly participate any further in the project.
The North American engineers moved the carburetor air intake from above to below the nose in order to accommodate the Merlin's updraft induction system. The intercooler radiator was added to the radiator group already located inside the scoop underneath the rear fuselage, and the ventral radiator group was made noticeably deeper than before and had a sharp-angled inlet standing more than two inches away from the underside of the fuselage. The matrix and door arrangement of the ventral radiator system were modified. Instead of the oil cooler being situated in the center of a circuular coolant radiator, it was relocated to the front of the duct and provided with its own ventral exit door. Further downstream, in a greatly enlarged duct, was the huge rectangular coolant matrix, with a much bigger exit door at the rear. The airframes were strengthened in order to make full use of the increased power available. New ailerons were fitted and the underwing racks were increased in capacity to take two 1000-lb bombs or their equivalent weight in drop tanks. A new four-bladed Hamilton Standard hydromatic paddle-bladed propeller was fitted. Provisions for fuselage- mounted guns were totally eliminated, plans being made for four 0.50-in machine guns mounted exclusively in the wings.
In August of 1942, 400 P-51Bs were ordered on the basis of NAA's performance estimates, even before the first example had flown.
The first XP-51B was flown by Bob Chilton on November 30, 1942. It was initially flown without armament. The performance improvement was nothing short of astounding. The XP-51B achieved a level speed of 441 mph at 29,800 feet, over 100 mph faster than the Allison-engined P-51 at that altitude. At all heights, the rate of climb was approximately doubled.
The USAAF now finally had an aircraft which could compete on equal terms with the Fw 190 and the later models of the Bf 109. The USAAF was finally fully sold on the Mustang, and a letter contract for 2200 P-51Bs was issued. The engine was to be the Packard V-1650-3, based on the Merlin 68. As 1943 dawned, the Mustang program suddenly expanded. Massive production of Merlin engines was to take place at both Packard at Detroit and at Continental at Muskegon. The huge Inglewood, California factory was greatly expanded and dedicated solely to Mustang production, with the B-25 Mitchell program being transferred to Kansas City. Production of the AT-6 series of trainers had earlier been transferred from Inglewood to a new plant built in great haste at Dallas, Texas. The USAAF instructed NAA to expand the Dallas plant even further as a second source for Mustangs. Inglewood-built Mustangs were designated P-51B, Dallas-built Mustangs were designated P-51C. These aircraft were almost identical, and can generally be distinguished only by serial number.
By the end of January 1943 the production standard for the P-51B/C had been decided. In order to take full advantage of the additional power, the airframe was re-stressed in detail and the aircraft was made capable of operating at considerably greater weights than was previously possible. The wing racks were eventually cleared to carry bombs of 1000 pounds each or a wide range of other stores including drop tanks or triple rocket tubes.
The engine installation was further refined, with a rectangular filtered-air inlet being added in each side of the carburetor duct, and the exhaust expelled through individual ejector stubs projecting through a slim fairing. The ailerons were modified aerodynamically and structural, although the changes were visible externally only by the fact that the tabs were made of plastic. The armament was to be four 0.50-inch Browning MG53-2 guns in the wings, with 350 rounds for each inner gun and 280 rounds for each outer gun. The fuselage nose guns were deleted.
The first P-51B flew on May 5, 1943, and the first P-51C flew on August 5 of that year. Inglewood built 1988 P-51Bs and Dallas built 1750 P-51Cs. The P-51Cs on the 1942 and 1943 budgets were given the company designation NA-103. 1350 NA-103s were built. Texas-built aircraft in the 1944 budget were designated NA-111.
Initially, the P-51B and C had the Packard V-1560-3 engine rated at 1400 hp for takeoff and 1450 hp at 19,800 feet and carried four 0.50-inch machine guns with a total of 1260 rounds. There were four hundred P-51B-1-NAs and 250 P-51C-1-NTs built.
In the pursuit of still more range, a P-51B was experimentally fitted with an extra 85 US gallon self-sealing fuel tank behind the pilot's seat, bringing the total fuel to 419 US gallons (including 2 drop tanks). Although the Mustang already offered outstanding range performance, this additional fuel made it even better. This extra range was being demanded by expanding operations in both the European and Pacific theatres. However, this extra fuel tank moved the center of gravity aft, which made the directional stability of the Mustang quite poor, so that the pilot would have to spend the first hour or so concentrating on keeping his airplane pointed in the right direction until this new tank was finally empty. The last 550 P-51B-5-NAs were fitted with this extra tank, becoming P-51B-7-NAs, and into P-51C-1-NTs, becoming P-51C-3-NT. In addition, some earlier P-51Bs and Cs were modified in the field to accommodate this tank. In service, however, the directional instability caused by the presence of a full fuel tank behind the pilot's seat was a hazard for new or inexperienced pilots, and the tank was usually restricted to 65 US gallons. This extra tank, nevertheless, still made a crucial difference in combat radius, and it was standard equipment in all future production versions. With this extra fuel, Mustangs were able to escort bombers all the way to Berlin from bases in Britain.
During the P-51B-10-NA and P-51C-1-NT production run, it was decided to omit the olive drab camouflage and to deliver the aircraft in their natural metal finish. The objective was now to try and bring the Luftwaffe into battle, not to hide from it. This move saved extra cost, weight, and drag.
With the introduction of the P-51C-5-NT onto the Dallas production line and the P-51B-15-NA in the Inglewood production line, the Packard V-1560-7 engine was adopted as standard. It offered 1450 hp for take off and a war emergency rating of 1695 hp at 10,300 feet. Maximum speed at 20,000 feet was reduced from 440 to 435 mph, but increased from 430 to 439 mph at 25,000 feet. 398 P-51B-10-NAs, 390 P-51B-15-NAs, and 1350 P-51C-10-NTs were built, all powered by the V-1650-7 engine.
A total of 91 aircraft from the Block-10 production lot (71 P-51B-10-NAs and 20 P-51C-10-NTs) were fitted with two oblique K24 cameras, or a K17 and a K22, to become F-6C-NA or -NT photo aircraft. Most of these aircraft retained their guns. In each case the cameras were mounted immediately in front of the structural break ahead of the tailwheel, looking out the left side.
The first combat unit equipped with Merlin-powered Mustangs was the 354th Fighter Group, which reached England in October of 1943. The 354th FG consisted of the 353rd, 355th and 356th Fighter Squadrons, and was part of the 9th Air Force which had the responsibility of air-to-ground attacks in support of the upcoming invasion of Europe. However, they were immediately ordered to support the bomber operations of the 8th Air Force. The 354th flew their first cross-Channel sweep mission on December 1, 1943, and scored their first victory on a mission to Bremen on December 16. However, inexperienced pilots and ground crews and numerous technical problems limited operations with the P-51B/C until about eight weeks into 1944. From the early spring of 1944, the Merlin-powered Mustang became an important fighter in the ETO.
The first P-51 ace was Major James H. Howard of the 354th Fighter Group. On January 11, 1944, he shot down five German fighters to become an "ace-in-a-day". He was awarded the Medal of Honor for this feat.
The 357th Fighter Group, also initially assigned to the 9th Air Force but was quickly transferred to operational control of the 8th Air Force for bomber escort. It flew its first P-51B escort mission on February 11, 1944.
The 363rd Fighter Group became the third P-51B operator in Europe on February 23, 1944.
The Fifteenth Air Force had been formed in November 1943 with three P-38 Lightning groups to escort Allied bombers. They were based in the Mediterranean theatre of operations. During April of 1944, Merlin-powered Mustangs began replacing Spitfires with the 31st and 52nd Fighter Groups, which transferred from the 12th to the 15th Air Force. The 31st flew its first mission on April 21, 1944, when its Mustangs escorted B-24s in an attack on the oil refineries at Ploesti in Romania.
The third fighter group to join the Fifteenth Air Force was the 332nd Fighter Group. It was a unit manned entirely by black airmen trained at Tuskegee, the Army being a segregated service in those days. They transitioned to the P-51C in June of 1944 while they were based at Foggia in Italy. The airmen of the 332nd FG had to spend nearly as much time battling segregation as they did the forces of the Third Reich. The top scorer was Lea Archer, with five air and six ground victories, although one aerial victory was later reallocated to another pilot to prevent him from becoming an ace. The proudest feat of the 332nd FG was that it never lost a bomber in its charge.
In March of 1944, Merlin-powered Mustangs accompanied B-17 Fortress and B-24 Liberator bombers all the way on a 1100-mile round trip to Berlin. The ability of escort fighters to accompany bomber formations all the way to their targets and still effectively counter intercepting Luftwaffe fighters after jettisoning their nearly empty drop tanks caused the German defenses no end of problems, and added considerable impetus to the American daylight bombing offensive.
Most of the P-51B/Cs were assigned to the 8th and 9th Air Forces in England, with a lesser number with the 12th and 15th USAAF in Italy. The P-51B/C remained the prime Mustang variant in service from December 1943 until March of 1944, when the bubble-topped P-51D began to arrive. However, P-51B/C fighters remained predominant until the middle of 1944, and remained in combat until the end of the war in Europe even after the arrival of large numbers of P-51Ds. Even as late as the last month of the war, 1000 out of the 2500 Mustangs serving in the ETO were of the P-51B/C variety.
P-51Bs and Cs were assigned to the following fighter groups in the European Theatre:
4th Fighter Group, 8th Air Force 20th Fighter Group, 8th Air Force 355th Fighter Group, 8th Air Force 339th Fighter Group, 8th Air Force 352nd Fighter Group, 8th Air Force 357th Fighter Group, 8th Air Force 359th Fighter Group, 8th Air Force 361st Fighter Group, 8th Air Force 479th Fighter Group, 8th Air Force 354th Fighter Group, 9th Air Force 363rd Fighter Group, 9th Air Force 52nd Fighter Group, 12th Air Force 325th Fighter Group, 15th Air Force 31st Fighter Group, 15th Air Force 332nd Fighter Group, 15th Air Force
In addition, P-38 fighter groups 20th, 55th and 364th FG, converted to a mixture of P-51B/C/D in Jul 1944.
Perhaps the best known P-51B aircraft is "Shangri La", a P-51B-5-NA (Ser No 44-6913) flown by the Fourth Fighter Group ace Don Gentile. Although the bubble-topped P-51D is far better known, the P-51B/C was actually the aircraft that turned the tide of the bomber war over Germany.
The Merlin-powered Mustang entered service in the China-Burma-India (CBI) theatre in September 1943. These aircraft were assigned to the 23rd and 51st Fighter Groups of the 5th Air Force. Early in 1944, the 311th Fighter Group of the 10th Air Force saw action in Burma with its Mustangs, flying in support of airborne troops attacking Japanese lines of communication. The top Mustang ace of the CBI theatre was Major John C. "Pappy" Herbst, with 18 kills.
In June 1944, the 10th Tactical Reconnaissance Group picked up the 12th and 15th Reconnaissance Squadrons, equipped with F-6B and F-6C photographic reconnaissance aircraft. F-6s served with the Eighth, Ninth, Twelfth, and Fifteenth Air Forces, and with the Fifth Air Force in the Far East. The F-6s retained their four 0.50-inch machine guns, and had frequent encounters with Luftwaffe fighters. Captain Clyde East of the 15th Squadron was the war's top-scoring reconnaissance pilot, with 15 aerial victories.
In late 1942, a deal was worked out between Britain and the USA in which Spitfire VBs would be transferred to the 8th Air Force in England, mainly for use as fighter-trainers. This cleared the way for Lend-Lease supplies to continue of the new Mustang model to the RAF. The RAF equivalent to the USAAF P-51B/C was the Mustang III. The RAF ultimately received 274 P-51Bs and 626 P-51Cs. A total of 59 Mustang IIIs were diverted to the Royal Australian Air Force and to other Allied air arms.
About 100 P-51Bs and Cs were supplied to the Chinese Air Force in 1943-44.
During late 1944, French units acquired some F-6Cs and began to operate them over Germany in January of 1945 on photo-mapping missions.
After the Mustang III aircraft had been delivered to England, the RAF decided that the hinged cockpit canopy offered too poor a view for European operations. A fairly major modification was made in which the original framed hinged hood was replaced by a bulged Perspex frameless canopy that slid to the rear on rails. This canopy gave the pilot much more room and the huge goldfish bowl afforded a good view almost straight down or directly to the rear. This hood was manufactured and fitted by the British corporation R. Malcolm & Co., and came to be known as the "Malcolm Hood". This hood was fitted to most RAF Mustang IIIs, and many USAAF Eighth and Ninth Air Force P-51B/C fighters received this modification as well.
In search of a more lasting solution to the problem of poor cockpit visibility from the P-51B/C, a P-51B (43-12101) was modified with a teardrop-shaped all-round cockpit canopy. Having proved that the concept was valid, two P-51B-10-NAs (42-106539/106540 were completed on the production line with teardrop-shaped bubble canopies and were redesignated P-51Ds. These became the prototypes for the famed P-51D series of Mustangs.
However, many pilots regarded the Malcolm-hooded P-51B/C as the best Mustang of the entire series. It was lighter, faster, and had crisper handling than the later bubble-hooded P-51D and actually had a better all-round view. Its primary weakness, however, was in its armament--only four rather than six guns, which often proved prone to jamming. Some of the modifications applied to the P-51D to improve the ammunition feed were later retrofitted into P-51B/Cs, which made their guns less prone to jamming. With modified guns and a Malcolm hood, the P-51B/C was arguably a better fighter than the P-51D, with better visibility, lower weight, and without the structural problems which afflicted the D. Its departure characteristics were also more benign.
Some 3740 P-51Bs and Cs were built. Some of the served with front-line units until the end of hostilities, but
others were converted as two-seat trainers or squadron hacks. The last P-51B passed out of service in 1949,
having been re-designated F-51B in 1948.
P-51C-10-NT 44-10947 spent the entire war stateside serving in training roles. At the end of the war, it was turned over to the Reconstruction Finance Corporation (RFC) facility at Searcy Field in Oklahoma. This was a government-run facility which was charged with the disposal of surplus military property. It was purchased by Paul Mantz and raced under the civilian registration NX1202. It won the transcontinental Bendix Air Race of 1946, a 2048 mile trip between Van Nuys, California to Cleveland, Ohio, at an average speed of 435.5 mph. On February 28, 1947, Paul Mantz set new wast-east record for a piston- engined aircraft by flying NX1202 from Burbank, California to LaGuardia Field, New York in 6 hours 7 minutes and 5 seconds. Mantz won the Bendix race again in 1947. He returned to California via LaGuardia, setting a new east-west record for this class of aircraft, reaching Burbank in 7 hours and 5 seconds. Paul Mantz won again in 1948, but this time he flew NX1204, another P-51C that he had purchased from the RFC.
In 1950, N1202 was sold to Capt. Charles F. Blair, who had plans for establishing a new round-the-world speed record. He named the plane *Stormy Petrel*, but soon changed it to *Excalibur III* On January 31, 1951, Captain Blair flew from New York International Airport to London in 7 hours and 48 minutes. The average speed was 450 mph, assisted by strong tailwinds. This broke the old New York-London record by 1 hour and 7 minutes. The record still stands for piston-engined aircraft. The next spring, Captain Blair then flew from London to Bremen, Oslo, and Bardufoss in Norway and then over the North Pole to Fairbanks, Alaska, landing there on May 29, 1951. On September 18,1952, President Harry S Truman presented Captain Blair with the Harmon International Trophy for his accomplishments.
Following these flights, Excalibur III was donated to the Smithsonian
Institution. Its sister ship, NX1204, was donated to the International Flight
and Space Museum in Santa Ana, California.
Although literally hundreds of P-51Ds are still in existence today (many of them still flying), very few P-51B/C fighters have survived to the present, even in museums. A few have been restored to flying status, including 42-106638, 43-12252, 42-103645, 42-103831, 43-25057, 43-25147, and 42-103293. 44-10947 (Excalibur III) is on display at the Udvar-Hazy Center at Dulles Airport, Virginia, and 43-24760 is in storage.
Serial numbers of P-51Bs:
43-12093/12492 North American P-51B-1-NA Mustang (NA-102) c/n 102-24541/24940. 400 aircraft 43-6313/7112 North American P-51B-5-NA Mustang (NA-104) c/n 104-22816/23305, 24431/24540, 24941/25140. 800 aircraft 43-7113/7202 North American P-51B-10-NA Mustang (NA-104) c/n 104-25141/25230. 90 aircraft 42-106429/106540 North American P-51B-10-NA Mustang (NA-104) c/n 104-25231/25342. 112 aircraft 42-106541/106738 North American P-51B-10-NA Mustang (NA-104) c/n 104-25343/25540. 198 aircraft 42-106739/106978 North American P-51B-15-NA Mustang (NA-104) c/n 104-25541/25780. 240 aircraft 43-24752/24901 North American P-51B-15-NA Mustang (NA-104) c/n 104-25781/25930. 150 aircraft total of 1990 P-51Bs Serial numbers of P-51Cs: 42-102979/103328 North American P-51C-1-NT Mustang (NA-103) c/n 103-22416/22765. 350 aircraft 42-103329/103778 North American P-51C-5-NT Mustang (NA-103) c/n 103-22766/22815, 103-25933/26332. 450 aircraft 42-103779/103978 North American P-51C-10-NT Mustang (NA-103) c/n 103-26333/26532. 200 aircraft 43-24902/25251 North American P-51C-10-NT Mustang (NA-103) c/n 103-26533/26882. 350 aircraft 44-10753/10782 North American P-51C-10-NT Mustang (NA-111) c/n 111-28886/28915. 30 aircraft 44-10783/10817 North American P-51C-11-NT Mustang (NA-111) c/n 111-28916/28950. 35 aircraft 44-10818/10852 North American P-51C-10-NT Mustang (NA-111) c/n 111-28951/28985. 35 aircraft 44-10853/10858 North American P-51C-11-NT Mustang (NA-111) c/n 111-28986/28991. 6 aircraft 44-10859/11036 North American P-51C-10-NT Mustang (NA-111) c/n 111-28992/29169. 178 aircraft 44-11037/11122 North American P-51C-11-NT Mustang (NA-111) c/n 111-29170/29255. 86 aircraft 44-11123/11152 North American P-51C-10-NT Mustang (NA-111) c/n 111-29256/29285. 30 aircraft total of 1750 aircraft
Specification of P-51B-1-NA:
One 1620 hp Packard Merlin V-1650-3 twelve cylinder Vee liquid-cooled engine. Maximum speed was 388 mph at 5000 feet, 406 mph at 10,000 feet, 427 mph at 20,000 feet, 430 mph at 25,000 feet, 440 mph at 30,000 feet. Range on internal fuel was 550 miles at 343 mph at 25,000 feet, 810 miles at 253 mph at 10,000 feet. With maximum external fuel, maximum range was 2200 miles at 244 mph. An altitude of 5000 feet could be attained in 1.8 minutes, 10,000 feet in 3.6 minutes, 20,000 feet in 7 minutes. Service ceiling was 42,000 feet. Weights were 6840 lbs empty, 9200 lbs normal loaded, 11,200 lbs maximum loaded. Wingspan was 37 feet 0 1/4 inches, length was 32 feet 3 inches, height 8 feet 8 inches, and wing area was 233 square feet.
Specification of P-51C-10-NT:
One 1695 hp Packard Merlin V-1650-7 twelve cylinder Vee liquid-cooled
engine. Maximum speed was 395 mph at 5000 feet, 417 mph at 10,000
feet, 426 mph at 20,000 feet, 439 mph at 25,000 feet, 435 mph at
30,000 feet. Range on internal fuel was 955 miles at 397 mph at
25,000 feet, 1300 miles at 260 mph at 10,000 feet. With maximum
external fuel, maximum range was 2440 miles at 249 mph. An altitude
of 5000 feet could be attained in 1.6 minutes, 10,000 feet in 3.1
minutes, 20,000 feet in 6.9 minutes. Service ceiling was 41,900 feet.
Weights were 6985 lbs empty, 9800 lbs normal loaded, 11,800 lbs
maximum loaded. Wingspan was 37 feet 0 1/4 inches, length was 32 feet
3 inches, height 8 feet 8 inches, and wing area was 233 square feet.