The P-51 Mustang is perhaps the most famous fighter of World War II and, many would agree, the best all-round piston-engined fighter produced by any of the combatants during that conflict. Total production of all Mustangs numbered 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 4,950 of the 10,720 air combat victories claimed by the AAF in Europe, and 4,131 of the 8,160 strafing claims made there, accounting for 48.9% of total losses inflicted on the enemy. They shot down more than 230 V-1 "buzz bombs," and even managed to score some kills against Luftwaffe jet fighters.

North American Aviation Inc (NAA) was formed on Dec 6, 1928 as a holding company with interests in a long list of prominent aircraft companies—Curtiss Aeroplane and Motor Co, Curtiss-Robertson, Curtiss-Caproni, Wright Aeronautical, Travel Air, North Aircraft, and Keystone. The prime mover behind that company was Clement M Keys, publisher of the Wall Street Journal.

    In 1929 Keys merged the two old rivals, Curtiss and Wright, to form the Curtiss-Wright Corp. All of those 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, acquiring Pitcairn, Transcontinental & Western Air (TWA today), Berliner-Joyce, and others faltering in the Great Depression.

    In 1933 General Motors Corp bought a sizeable interest in North American Aviation Inc. It also acquired General Aviation Corp (which had taken over Fokker Aircraft) and Dayton-Wright. GMC's Ernest R Breech was appointed as president of North American Aviation, and the companies were grouped into two large conglomerates—General Aviation Mfg Corp (comprised of Berliner-Joyce, General Aviation, and Curtiss-Caproni) and the Sperry Corp (Sperry Gyroscope, Ford Instrument, Curtiss-Wright, and Intercontinent Aviation). Sperry was disposed of a few months later, and the Air Mail Act of 1934 made it illegal for aircraft manufacturing industries to have controlling interests in airlines, so North American Aviation was forced to divest itself of its shares in Eastern and TWA.

    At that time Breech handed over management of the company to James H "Dutch" Kindelberger, who had an extensive background in aviation both as a pilot and aeronautical engineer. He accepted the job on the condition that friend and colleague, John Leland "Lee" Atwood, could join him as vice-president. Under their 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 MD and another factory being opened in Jan 1936 at Inglewood, near Mines Field (now part of LAX).

    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 soon be coming. Once 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 Atwood to formulate an outline for a fighter project. A project team was formed at NAA, made up of such people as Raymond H Rice, Edgar Schmued, Larry Waite, and Ed Horkey. (A sort-of urban legend had grown up about Edgar Schmued that claimed 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. Bell P-39s and Curtiss P-40s 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 was NAA, who had already been building NA-16 trainers for the RAF as the Harvard. In April of 1940 Kindelberger was requested to manufacture Curtiss P-40Ds under license for the RAF. He responded that NAA could do that, but countered that 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 Apr 10, 1940, 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 added a condition—two examples of the initial British NA-73 lot were to be transferred to the AAC 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 had 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 s/ns were issued [AG345/664] and the aircraft given the name Mustang I for 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 XP-46. Although NAA did pay $56,000 to Curtiss for technical aerodynamic data for the XP-46, there was only a very broad resemblance between it and the NA-73X. The Curtiss shared only a similar radiator/oil-cooler configuration and did not have laminar-flow wings. In fact, development of XP-46 lagged behind that of NA-73X, with prototypes not ready for flight until Feb 1941.

    As well, 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 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-skin structure, with a 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 .50 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, as a 50% chord rather than the usual 20%. Those profiles had little camber, the undersurface being almost a mirror image of the upper. The 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.

    Wing structure was designed to be as simple and easy to construct as possible. Leading and trailing edges were straight lines to the extent possible, and the underlying structure was simple to manufacture, being left and right halves joined at the centerline.

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

The British had also specified that a liquid-cooled inline engine be used, and the 12-cylinder Allison V-1710 was the only American-built engine that fit the bill. It was a little bigger and lighter than the Merlin and similar in power at low altitudes; however, at higher altitudes it suffered from a rapid drop in power than the Merlin. NAA briefly considered using a turbo-supercharger to improve high-altitude performance, but ruled against it in light of a tight schedule.

    The Allison 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 under the rear fuselage in a streamlined duct. The duct actually contributed some propulsive thrust by adding heat energy to the incoming air and expelling it out the back at a higher velocity. Drawbacks of such a cooling arrangement were extra weight and added combat vulnerability of the long pipes that led to and from the engine.

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

    At British insistence, armament was somewhat heavier than American standards of the day. Two .50 Browning M2 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 .50s were mounted upright inside the wings, outboard of the landing gears. Four .30 Brownings were mounted further outboard on the wing, with each inboard gun 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 Sep 9, 1940. As agreed, the fourth and tenth production NA-73s would be diverted to the Army, where their designation XP-51 was to be assigned. 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, 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 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. Those markings were retouched out in photos of the aircraft, and only later was the civil registration [NX19998] applied and the engine hood ahead of the cockpit painted anti-glare black.

    The reason for delay in engine delivery was because it was "government-supplied equipment" 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 then rolling off production lines. The engine eventually installed was a non-supercharged 1100hp V-1710-F3R. Test pilot Vance Breese flew the NA-73X for its first time on Oct 26, 1940. It was a clear 25 mph faster than the P-40, despite being powered by the same engine.

Following tests there were several changes in the geometry of the ventral ducting and 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 Nov 20, 1940, while on its fifth flight, test pilot Paul Balfour forgot to change fuel tanks, ran out of gas, and NA-73X suffered a forced landing, ending up on its back in a farm field. While the mishap put it out of action for several months, since the accident was not the fault of the aircraft itself, it did not unduly delay the program. The NA-73X resumed flying on Jan 11, 1941 and continued in the initial development program until being retired on July 15, 1941.

    In Dec 1940 the RAF ordered 300 more Mustangs [AL958/AL999, AM100/AM257, AP164/AP263], designated NA-83 by the factory, differing from NA-73s only in having broad fishtail ejector exhausts.