North American NA-73

Last revised May 31, 2022

The P-51 Mustang was perhaps the most famous fighter of World War II, and, many would say, the best all-round piston-engined fighter produced by any of the combatants during that conflict. Total production of all Mustangs amounted to 15,575 in the USA and 100 in Australia, ranking only behind the P-47 Thunderbolt in being the fighter manufactured in greatest numbers for the USAAF. Mustangs accounted for 4950 of the 10,720 air combat victories claimed by the USAAF in Europe, and 4131 of the 8160 ground strafing claims made in the same theatre, accounting for 48.9 percent of total losses inflicted on the enemy. They shot down more than 230 V-1 "buzz bombs", and they even managed to score some kills against Luftwaffe jet fighters.

North American Aviation, Inc had been formed on December 6, 1928 as a holding company with intesests in a long list of prominent aircraft companies--Curtiss Aeroplane and Motor Company, Curtiss- Robertson Airplane Manufacturing Company, Curtiss-Caproni Corporation, Wright Aeronautical, Travel Air, North Aircraft Corporation, and Keystone Aircraft Corporation. The prime mover behind this holding company was Clement M. Keys, who had run the *Wall Street Journal* newspaper. In 1929, Keys merged the two old rivals Curtiss and Wright to form the Curtiss-Wright Corporation. All of these various companies managed to retain their own separate identities and continued to function as more-or-less independent entities while Keys continued to expand his empire. Sperry Gyroscope Co., Inc, Pitcairn Aviation Inc (which started Eastern Air Transport in 1930), Berliner-Joyce Aircraft Corporation, Ford Instrument, Transcontinental and Western Air (the TWA of today), Intercontinent Aviation Inc, and a substantial interest in the Douglas Aircraft Corporation were all picked up fairly cheaply during the early years of the Depression.

In 1933, General Motors bought a sizeable interest in North American Aviation, Inc. In addition, it acquired the General Aviation Corporation (which had taken over the Fokker Aircraft Corporation) and the Dayton- Wright Corporation. Ernest R. Breech of General Motors was appointed as president of North American Aviation, Inc., and the companies were grouped into two large conglomerates: General Aviation Manufacturing Corporation (which comprised Berliner-Joyce, General Aviation, and Curtiss Caproni) and the Sperry Corporation (which comprised Sperry Gyroscope Co., Inc, Ford Instrument, Curtiss-Wright Corporation and Intercontinent Aviation).

However, control of Sperry Corporation was disposed of a few months later. In addition, the Air Mail Act of 1934 made it illegal for aircraft manufacturing industries to have controlling interests in airlines, and North American Aviation, Inc.was forced to divest itself of its shares in Eastern and TWA. At this time Ernest R. Breech handed over the management of the company to James H. "Dutch" Kindelberger, who had an extensive background in aviation both as a pilot and as a degreed aeronautical engineer. Kindelberger accepted the job under the condition that his friend and colleague John Leland "Lee" Atwood could join him as vice-president. Under Kindelberger's leadership, North American Aviation, Inc. became a "real" aircraft manufacturing concern rather than just a holding company, with General Aviation becoming the manufacturing arm with factories at Dundalk, Maryland and another factory being opened up in January 1936 at Inglewood, California near Mines Field (now part of the Los Angeles airport). Not only was Kindelberger an excellent businessman, he was also a capable aeronautical engineer. As early as 1938, Kindelberger had made numerous trips to Europe seeking orders for his company, and he had the opportunity to see up close some of the airplanes that would be in combat in the war that almost everyone believed would shortly be coming. After hostilities broke out, Kindelberger eagerly sought out combat reports from both sides and developed some ideas of his own.

Although Kindelberger had no experience with fighters, he collaborated with his friend and colleague J. Leland Atwood to formulate an outline for a fighter project. A project team was formed at North American, made up of such people as Raymond H. Rice, Edgar Schmued, Larry Waite and E. H. Horkey. A sort of urban legend has grown up about Edgar Schmued, which claims that he had once worked for Willy Messerschmitt and that the Mustang was heavily influenced by the Bf 109.

Following the outbreak of war in Europe, the British Purchasing Commission, headed by Sir Henry Self, was posted to New York to determine if American combat aircraft could be of any use to the Royal Air Force. The Bell P-39 Airacobra and the Curtiss P-40 Warhawk were ordered in substantial numbers, even though they were not up to the performance standards of the latest British and German fighters.

One of the corporations that Self had contacted had been the North American Aviation corporation. North American had already been building NA-16 trainers, and the British ordered a number of them for the RAF as the *Harvard*. In April of 1940, Kindelberger was summoned by the British Air Purchasing Commision and asked to manufacture the Curtiss Hawk 87 (P-40D) under license for the RAF. Kindelberger responded that NAA could do that if it were really required, but countered that he and his company could build a better fighter than the P-40 and that they could design a REAL fighter in the same time that it would take to put the P-40 into production. The British commission felt that they could take Kindelberger at his word, and on April 10, 1940 they accepted his proposal on the condition that the first prototype be ready in 120 days. The design was assigned the company project name of Model NA-73.

At that time, the USAAC reserved for itself the right to block any foreign aircraft sales that it regarded as not in the Army's interest, for whatever reason. On May 4, 1940, the US Army reluctantly agreed not to block the British sale, but they added a condition. Two examples of the initial NA-73 lot for Britain were to be transferred to the USAAC for testing free of charge.

The NA-73X prototype contract was signed on May 23, 1940. The British insisted that a heavy eight-gun armament be fitted. NAA had actually been quietly working on such a fighter project since the summer of 1939, and by that date they had actually already completed much of the detail design. On May 29, a provisional RAF procurement was issued for 320 aircraft, contingent on satisfactory testing of the prototype. NAA agreed to start deliveries in January 1941. RAF serial numbers were to be AG345 through AG664, and the aircraft was given the name Mustang I in RAF service.

Another urban legend surrounding the Mustang is that it owed a great deal to the Curtiss XP-46 and, in fact, stole numerous design features from that fighter. It is true that the British had insisted that since NAA had no fighter experience they should secure all current data from Curtiss about both the P-40 and the XP-46. Although NAA did pay $56,000 to Curtiss for technical aerodynamic data on the XP-46, there was only a very broad resemblance between the XP-46 and the NA-73X. The Curtiss aircraft shared only a similar radiator/ oil-cooler configuration with the NA-73X, and did not have laminar flow wings. In point of fact, the development of the XP-46 lagged behind that of the NA-73X, and prototypes were not ready for flight until February of 1941. In addition, preliminary design of the NA-73X was completed before NAA gained access to the Curtiss material. It could even be argued that the XP-46 data was most useful to NAA in guiding them in what NOT to do. The NA-73X appears to owe virtually nothing to any previous fighter design. Nevertheless, despite convincing denials from both Edgar Schmued and aerodynamicist Edward Horkey, the full magnitude of the contribution of Curtiss to the NA-73X design remains controversial to this day.

The NA-73 featured an all-metal stressed-ski structure, with the wing having a sheet-web main spar and an almost equally strong rear spar to carry the ailerons and the flaps. Special attention was paid to features which would make the aircraft simple and inexpensive to manufacture. The two wing spars had to be far enough apart to accommodate the length of a 0.50-in machine gun, with only the barrel protruding ahead of the main spar. Most previous NAA aircraft had left and right wings bolted to a horizontal center section, but the Mustang had the wings meeting on the centerline, with dihedral emanating from that line.

A special NACA laminar flow wing profile was adopted for the Mustang. This was an aerofoil which had a thickness that kept on increasing far beyond the usual location, i.e., to 50 percent chord rather than the usual 20 percent. These profiles had little camber, the undersurface being almost a mirror image of the upper. This wing was much more "slippery" than the old profiles, and provided lesser aerodynamic drag at high speeds than did more conventional aerofoils. However, it also had less lift at low speeds, so the NA-73X had to have large and powerful flaps to keep landing speeds from being impractically high.

The wing structure was designed to be as simple and as easy to construct as possible. The leading and trailing edges were straight lines to the extent possible, and the underlying structure was simple and easy to manufacture. The wing was made in left and right halves that were joined at the centerline. Each wing had two straight spars that were far enough apart for a 0.5-inch Browning machine gun to fit between them with only the barrel projecting through the front spar.

The main landing gear members had a track of almost 12 feet, which made landing much easier than in such fighters as the Spitfire and Bf 109. The main wheels retracted inward into wheel wells in the wing forward leading edge, the leading edges being kinked forward at the fuselage join to provide sufficent room. The retracted wheels were covered by doors hinged near the aircraft centerline, and were closed again by their own jacks when the landing gear was fully extended. The tailwheel was fully retractable into a compartment with twin doors. The tailwheel was steerable and was linked to the rudder.

The British also specified that a liquid-cooled inline engine be used, and the Allison V-1710 twelve-cylinder Vee was the only American-built engine which fit the bill. The Allison V-1710 was a little bigger than the Merlin, slightly lighter, and similar in power at low altitudes. However, at higher altitudes the Allison suffered from a rapid drop in power in comparison to the Merlin. NAA briefly considered using a turbosupercharger to improve high-altitude performance, but ruled against it on the grounds of a tight schedule.

The Allison engine had a downdraft carburetor, so the ram inlet of the NA-73X was located above the cowling. Radiators for cooling the ethlyene glycol and lubricating oil were located in a single heat-exchanger installed underneath the rear fuselage in a streamlined duct. The duct actually had the ability to add some propulsive thrust, by adding heat energy to the incoming air and expelling it out the back at a higher velocity. The drawbacks of such a cooling arrangement were the extra weight and the added combat vulnerability of the long pipes that led to and from the engine.

Fuel was housed in two self-sealing tanks housed in the wing spars, one in each inboard wing. Total capacity was 180 US gallons, almost twice the fuel capacity of a Spitfire.

At British insistence, armament was somewhat heavier than American standards of the day. Two 0.5-inch M2 Browning machine guns were installed in the underside of the nose beside the engine crankcase, synchronized to fire through the propeller arc. The left gun was staggered ahead of the right in order that the magazines could lie one behind the other. Two 0.50-inch guns were mounted upright inside the wings, outboard of the landing gears. Four 0.30-inch Browning machine guns were mounted further outboard on the wing, with each inboard 0.30-inch gun being mounted lower so that its muzzle was below the leading edge. Ammunition for all the wing guns was in three long spanwise boxes outboard of the guns.

Final assembly and engine installation began on September 9, 1940, 102 days after the initial British order.

In a contract approved on September 20, 1940, it was agreed that the fourth and tenth production NA-73s would be the planes diverted to the Army. The designation XP-51 was to be assigned to these two planes.

On September 24, 1940, the RAF increased their Mustang I order to 620 planes.

The NA-73X prototype emerged from Inglewood plant in only 102 days, thus meeting the 120-day deadline with time to spare, although the airplane rolled out of the factory without an engine, which had been delayed at the Allison factory. In the absence of the new disk brakes, the aircraft was rolled on wheels borrowed from an AT-6 trainer. It was completely unpainted except for six aperture shapes painted on the wing leading edges to show where the guns would be installed. These aperture shapes were retouched out in many reproductions of the most famous photographs of the aircraft. Only later was the civil registration NX19998 applied and the fuselage ahead of the cockpit painted with anti-dazzle black.

The reason for the delay in engine delivery was because it was "government-furnished equipment" that was furnished on an as-available basis. Since the NA-73X was a private venture it was not allocated a very high priority in comparison with P-40s that were then rolling off the production lines. The engine that was eventually installed was an un-turbosupercharged Allison V-1710-F3R liquid-cooled Vee, rated at 1100 hp.

Veteran test pilot Vance Breese flew the NA-73X for the first time on October 26, 1940. Weights were 6278 lbs empty, 7965 lbs normal loaded. It was a clear 25 mph faster than the P-40, even in spite of being powered by the same engine.

Following tests, there were several changes in the geometry of the ventral ducting and the controllable flaps. By the time that the NA-73 had been cleared for production, the duct had had its inlet moved downward so that its upper lip was lower than the underside of the wing, thus avoiding the ingestion of a turbulent boundary layer of air into the radiator cooler.

On November 20, 1940, while on the fifth test flight of the NA-73X, test pilot Paul Balfour forgot to change fuel tanks, ran out of gas, and suffered a forced landing. The plane ended up on its back in a farmer's field. This mishap put the prototype out of action for several months. However, since this accident was not the fault of the aircraft itself, this did not unduly delay the program. The NA-73X aircraft resumed flying on January 11, 1941 and continued in the initial development program until being retired on July 15, 1941.

In December 1940, the RAF ordered 300 more of the Mustang Is which embodied only minor modifications. These were designated NA-83 by the factory. RAF serials were AL958/AL999, AM100/AM257, and AP164/AP263. They differed from the NA-73s only in having broad fishtail ejector exhausts. Sources:

  1. American Combat Planes, Ray Wagner, Third Enlarged Edition, Doubleday, 1982.

  2. The American Fighter, Enzo Angelucci and Peter Bowers, Orion Books, 1987.

  3. War Planes of the Second World War, Fighters, Volume Four, William Green, Doubleday 1964.

  4. United States Military Aircraft since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian, 1989.

  5. Fighting Mustang: The Chronicle of the P-51, William N. Hess, Doubleday, 1970.

  6. Classic Warplanes: North American P-51 Mustang, Bill Gunston, Gallery Books, 1990.

  7. Famous Fighters of the Second World War, Volume I, William Green, 1967.

  8. British Military Aircraft Serials, 1912-1969, Bruce Robertson, Ian Allen, 1969.