In early 1943, joint discussions were held between British authorities and North American Aviation dealing with the subject of what the next generation of Mustangs should look like. The original NA-73 had been designed to higher load factors than the British Air Purchasing Commission had required. As a result, the structure of the Mustang was considerably heavier than that of the Spitfire, and it was felt that a considerable improvement in performance might be obtained if structural weight could be reduced. Edgar Schmued that traveled to England and had inspected the Supermarine factory, and he had also studied captured Messerchmitt and Focke-Wulf fighters. In January of 1943, North American Aviation suggested to the USAAF that they build a special lightweight version of the Mustang. It was agreed that a thorough redesign would be carried out, mainly to reduce weight but also to simplify systems, improve maintenance, and enhance performance without changing the engine. The new Mustang was to be designed to a combination of optimal British and American strength requirements, but mainly to those laid down in British Air Publication 970.
The project was given the company designation NA-105. Two prototypes were ordered under the designation XP-51F, the contract being amended in June of 1943 to cover the purchase of five XP-51Fs, all powered by Packard V-1650-3 engines. Serial numbers were 43-43332/43336. The British Air Commission requested that two of these aircraft be given to then for evaluation. A request by the Technical Command for the procurement of twenty-five service test P-51Fs was not authorized, since it was felt that prototype trials should be made before any quantity production was undertaken.
Resemblance to the previous Mustang was only coincidental, since the structure of the aircraft was almost completely redesigned and almost no parts were common. Most of the changes were made in an attempt to save weight. The main landing gear members were redesigned and the wheels and tires were greatly reduced in size. New disc brakes were fitted to the wheels. The wing was slightly larger in area, and had a straight-line leading edge, completely eliminating the familiar "kink" of the earlier Mustang versions. The wing aerofoil was changed to an even newer low-drag "laminar flow" profile. The inboard wing guns were deleted, the remaining four guns having 440 rounds each. The two wing tanks were reduced in capacity to 102 US gallons each, and the fuselage tank was eliminated entirely. The engine mounting was simplified, the "integral" engine cradle for the V-1650-7 saving over 100 pounds of weight and improving the access to the engine. The hydraulic system was simplified and increased in pressure. The engine coolant and intercooler radiators were redesigned and installed in a completely new duct which had a vertical inlet which was placed even farther away from the underside of the wing. The oil cooler was removed from the rear radiator group, enabling the latter to be made smaller and making it possible to eliminate the long and vulnerable oil pipes. The oil was passed through a heat exchanger mounted on the front of the oil tank and next to the engine intercooler. The flow of glycol carried away the heat from the oil. The cockpit layout was improved (with the standard British panel being adopted), and the pilot's back armor was made integral with the seat. The canopy was made much larger in an effort to reduce the drag still further. Aerodynamic control surfaces were improved, and the tail surfaces were made larger. The ailerons were given a larger degree of movement, and the chord of the flaps and the ailerons were made equal. Still more weight was saved by using a three-bladed Aeroproducts hollow-steel propeller. Many minor metal parts were replaced with molded plastic parts.
Before construction began, it was agreed that the last two of the NA-105 airframes would be fitted with Rolls Royce Merlin 145M engines obtained from England under reverse Lend/Lease. These aircraft were designated XP-51G and bore the serial numbers 43-43335/43336.
Engineering inspections were held in February 1944. The first XP-51F was flown by Bob Chilton on February 14, 1944. The second and third XP-51F flew on May 20 and 22 of that year. Equipped empty weight was about 2000 pounds less than that of the P-51D, and combat weight was 1600 pounds less. The engine was the Packard Merlin V-1650-7 engine of 1695 hp, same as the powerplant of the P-51D. Considering that the equipped empty weight was about a ton less than that of the P-51D, the performance improvement was not as spectacular as might have been anticipated-- maximum speed was 466 mph at 29,000 feet.
Work on the conversion of the fourth and fifth NA-105 airframes as XP-51Gs began in January 1944, with the Merlin 145M engines arriving in February. Five-bladed propellers were fitted, but the XP-51G was otherwise similar to the XP-51F. The date of the first flight of the XP-51G is a matter of some dispute--most sources claim that first XP-51G was flown by Ed Virgin on August 10, 1944, but the manufacturer credits Bob Chilton with the first flight on August 12, while other s claim that Joe Barton may have taken the XP-51G up for the first time on August 9. The second machine followed on November 14. The engine was the Rolls-Royce Merlin 145M engine rated at 1910 hp., driving a Rotol propeller with five wooden blades (almost identical to the propellers of the Spitfire XIV). However, the XP-51G flew only once with the five-bladed propeller during a 20-minute flight, and all other flying was carried out with a more conventional Aeroproducts Unimatic A-542-B! four-bladed propeller. It was readily apparent that this was the hottest Mustang yet-- maximum speed was 472 mph at 20,750 feet.
The third XP-51F was shipped to the United Kingdom on June 20, 1944 after preliminary flight checks. It was painted in RAF camouflage and was named Mustang V. The RAF serial number was FR409. The A&AEE at Boscombe Down found the Mustang V to weigh only 7855 pounds in interceptor trim. They rated it very highly except for a severe lack of directional stability which required frequent heavy application of rudder in certain flight conditions.
The second XP-51G was shipped to the United Kingdom in February 1945. This plane was also named Mustang V, and bore the RAF serial number FR410. It is widely reported to have achieved a speed of 495 mph during tests at the A&AEE at Boscombe Down in February 1945, although NAA claimed only 472 mph for the other G at the same altitude. However, by this time RAF priorities had changed, and no further flight testing took place. The fate of FR410 after the end of test flying is uncertain.
Neither the XP-51F nor the G ever proceeded any further than the prototype stage. The Merlin 100-series of engines had not quite reached the stage where they were fully ready for production. In the meantime, the Packard Motor Car Company had set up its own development program and had come up with the V-1650-9 version of its license-built Merlin, capable of delivering a war emergency power of 1900 hp at 20,000 feet with water/alcohol injection. Packard said that they could deliver this new engine starting in late 1944. Consequently, this engine was chosen to power the next production Mustang, which was designated P-51H. Neither the P-51F nor the G were developed any further, although the work on these two airplanes was invaluable in the development of the P-51H.
The last prototype in the lightweight NA-105 series was the XP-51J, which was similar to the F and G models except that it reintroduced the Allison V-1710 engine to bring the Mustang full circle. The Allison engine was, however, the V-1710-119 version with a two stage, gear-driven supercharger, rated at 1500 hp for takeoff and 1720 hp with water injection at 20,700 feet. Unlike earlier Allisons, this engine had an updraft carburetor. The nose geometry was substantially modified, and all air inlets in the nose were completely eliminated. Instead, the carburetor air was taken in through a ram inlet at the front of the radiator duct and piped to the engine. A dorsal fin was fitted.
Two XP-51J prototypes were ordered, with serial numbers being 44-76027 and 44-76028. 44-76027 made its first flight on April 23, 1945, piloted by Joe Barton. The XP-51J weighed 6030 pounds empty and 7550 pounds normal loaded. It was anticipated that a maximum speed of 491 mph could be achieved at an altitude of 27,400 feet, but this was never achieved during tests because the new Allison had not yet been cleared for full power operations. XP-51J Ser No 44-76027 was, in fact, loaned to Allison so that they could use it to iron out the bugs in their engine. The other XP-51J prototype, Ser No 44-76028, was never actually flown, but was scavenged for spare parts to keep the other example flying. The end of the war in the Pacific brought all further work on the XP-51J to an end.
It is an odd fact that no inflight photos were ever taken of the XP-51F, G, or J. It seems that pilots and other people were too busy with wartime testing to schedule photo sessions. In later years, historians have looked in vain for a photographic record of these lightweight Mustangs in the air.
However, XP-51G 43-43335 has survived all these years. In the 1980s, John Morgan of La Canada, California attempted to restore and fly this aircraft. Unfortunately, the parts of the XP-51G are not interchangeable with those of the far more numerous P-51D, and the aircraft has a different center of gravity.
Specification of XP-51F
466 mph at 29,000 feet, and an altitude of 19,500 feet could be reached in 4.9 minutes. Service ceiling was 42,500 feet. Normal range was 650 miles, and maximum range was 2100 miles. Weights were 5635 lbs. empty, 7610 lbs. normal loaded, and 9060 lbs. maximum. Wingspan was 37 feet 9 1/4 inches, length was 32 feet 2 3/4 inches, height was 8 feet 8 inches, and wing area was 233 square feet.
Specification of XP-51G:
One Rolls-Royce Merlin 145M engine rated at 1910 hp., driving a maximum speed was 472 mph at 20,750 feet, and an altitude of 20,000 feet could be reached in 3.4 minutes. Service ceiling was 45,700 feet. Normal range was 485 miles, and maximum range was 1865 miles. Weights were 5750 lbs. empty, 7265 lbs. normal loaded, and 8885 lbs. maximum. Wingspan was 37 feet 9 1/4 inches, length was 32 feet 2 3/4 inches, height was 8 feet 8 inches, and wing area was 233 square feet.
Specification of XP-51J:
One Allison V-1710-119 liquid-cooled eigine with a two stage,
gear-driven supercharger, rated at 1500 hp for takeoff and 1720 hp
with water injection at 20,700 feet.