Today marks the 75th anniversary of the first flight of one of the most legendary fighters  of all time, the North American P-51 Mustang. There are many myths around the birth of the P-51, from it being designed and built in120 days to the claim that much of it was plagiarized from Curtiss Wright to even the origins of its name.

In this excerpt from Allison-Engined P-51 Mustang (Air Vanguard 1) author Martyn Chorlton tells the history of the P-51’s birth.

The NA-73X—birth of a legendary fighter
Sir Henry Self of the British Purchasing Commission [BDPC] already had a good working relationship with North American’s ‘Dutch’ Kindelberger in terms of Harvard production, which was extended to more aircraft on the outbreak of war. With regard to the P-40, NAA was already at full capacity producing the fighter for the USAAC. This would mean that it would be several months before any P-40s reached Britain, so the BDPC had no alternative but to look for a second source of production. Coincidentally, Kindelberger offered the BDPC its latest new twin-engined bomber, which would go on to become the highly successful B-25 Mitchell. At the same time, in an effort to divert more P-40s to Britain, a delegation led by Self in January 1940 proposed NAA could build the Curtiss fighter under license for the RAF. Rather than reject the notion out of hand, NAA responded with a remarkable proposal. After discussing various ideas, Kindelberger approached his chief designer, Edgar Schmued, and said, “Ed, do we want to build P-40s here?” Schmued, who had been waiting for this opportunity, responded by saying, “Well, Dutch, don’t let us build an obsolete airplane, let’s build a new one. We can design and build a better one.” This statement was partly prompted by the problems NAA foresaw in adapting the P-40 to its own production methods, and, possibly more significantly, by personal pride in the company’s ability to design a better aircraft.

NA-73XTime was of the essence, as Kindelberger was worried that Britain could not afford to wait. Kindelberger and his vice-president, British-born John Leland Atwood, travelled to New York to present Self with NAA’s proposal. In typical no-nonsense style, Kindelberger described how NAA would design a single-seat fighter from the ground up, using the same Allison engine fitted in the Curtiss fighter. The most significant difference between the two fighters would be that the NAA machine would have superior performance because of an extremely low-drag airframe. NAA would also design the aircraft for current mass production techniques to ensure a rapid build.

The BDPC was most impressed with the proposal and gave Kindelberger the go-ahead for a preliminary design study. This was the only moment of caution by the BDPC, which was aware of NAA’s inexperience in the design and mass production of a single-seat fighter. NAA’s only previous experience was the application of a more powerful engine and a few machine guns to a heavily modified Harvard, at the same time as conversion to a single-seat model. The BDPC’s decision to take the idea to design study stage displayed a lot of confidence in Kindelberger.

NAA designs and builds a fighter
The ball started rolling almost instantly after Kindelberger sent a telegram back to his designers at Inglewood on April 24, 1940. Chief of engineering Raymond Rice and assistant chief design engineer Edgar Schmued both immediately set their respective teams to work, despite it being a Saturday.

Kindelberger caught a flight back to California, leaving Atwood behind in New York to continue negotiations. At Inglewood, the design teams continued working through the night on general arrangement drawings and a preliminary weight study, all in time for Kindelberger to view the results by 10 am on April 25. The design at this stage was provisionally known as the NA-50B. As promised, the drawings were delivered to Kindelberger’s office and the president of NAA was very happy with what he saw. The Schmued-produced drawings presented a very sleek, low-wing monoplane with every effort incorporated to keep drag to a minimum. The design was simple yet functional and Kindelberger felt confident that the British would be pleased.

The new aircraft would be built around the Allison V-1710 water-cooled 12-cylinder inline Vee engine, which was already used in the P-40, but with greater range and performance. In 1938, Kindelberger was lucky enough to visit both the Heinkel and Messerschmitt factories, taking detailed notes on the production of liquid-cooled engined fighters. John Atwood was also ordered by the British to obtain (at a cost of $56,000) as much technical aerodynamic information as possible about Don Berlin’s Curtiss XP-46. It was this data that has caused a great deal of unnecessary conjecture as to the design origins of the Mustang. NAA never made a secret of the purchased material and it has also transpired that the USAAC insisted that the company should have the data at its disposal as well. However it is very unlikely that any information gleaned from the XP-46 as a whole would have been of any practical assistance. The NA-73X was destined to leave the NAA as a considerably more advanced aircraft that the Curtiss machine. The Curtiss XP-46 was a scaled-down version of the P-40, incorporating many unique features such as an inward-retracting undercarriage, slotted wings, self-sealing fuel tanks, armor protection for the pilot and a radiator set below the fuselage directly under the cockpit. It was the latter feature that, externally, was the only similarity between the Curtiss design and the new NAA machine.

Atwood made several trips to the BDPC offices and was heavily involved in the negotiations, as described by the man himself.

I made it clear [to the British] that we had no design, but that if authorized to proceed, we would design and build the aircraft in accordance with the representations I had made to the BDPC. These conversations went on until about the last week in March or the first week in April, when apparently affirmative recommendations were made to Sir Henry Self. At that time he called me in and discussed the project and asked me for a definite proposal. He made a reservation, however, and took note of the fact that we had not ever designed an actual fighter plane. He asked me if I thought I could get copies of the wind-tunnel tests and flight tests of the P-40 airplane. He said if I could, it would increase their confidence in our ability to move forward in a timely way.

I told him I would try, and that night I took a train to Buffalo where I called upon Mr Burdett Wright who was general manager of the Curtiss division at Buffalo. After negotiating with him for most of the day, I arranged to purchase copies of the wind-tunnel tests and the flight test report for the sum of $56,000, which would cover the out-of-pocket expenses and some proportion of the cost of the tests. I went back to New York and indicated to Sir Henry that I had been able to secure the data and presented him with a draft of the letter contract, which called for the production of 320 NA-73 aircraft equipped with an Allison engine and certain armaments to be furnished by the British, and an airframe to be designed and built by NAA—the total cost to the British government excluding engine, armaments, etc., was not to exceed $40,000 per airplane.

Although some technical work was by then being done in Los Angeles, we had not at this time presented the British Purchasing Commission President of the North American Aviation Company, James H. “Dutch” Kindelberger (left), and his vice-president, John Leland Atwood, in formal pose in 1942. “Dutch” became NAA’s president and general manager in 1934 when the company was first formed. Later promoted to chairman in 1948, he remained in this position until his death in July 1962. (NAA)[BDPC] with drawings or specifications of any kind except for free-hand sketches I had used to demonstrate the concept in informal conversations, and a letter contract was the sole document available. Sir Henry Self executed this document, after having it edited by his legal staff, and with this instrument the Mustang project got under way.

The blueprints and design specifications of the new NA-73X were enthusiastically approved by the BDPC. The design was actually shown to the British before the data from the XP-46 was thoroughly analyzed. Despite this, the Curtiss design team would accuse NAA of plagiarism, even though it was obvious that the XP-46 had its roots in the early 1930s while the NA-73X utilized many new design features.

When the British approved the design of the NA-73X, the timescale of 120 days is often quoted. This timescale is and has been subject to much discussion. Whether it was from approval of the drawings to prototype rollout, or from the date of the initial order to first flight, or a combination of the two, is unknown, but it certainly was not applied by the BDPC. The preliminary design was approved by Sir Henry Self, Air Vice-Marshal G. B. A. Baker, and Mr H. C. B. Thomas of the Royal Aircraft Establishment (RAE) on May 4, 1940, and the first order for 320 aircraft was placed on May 29, with a price of $33,400 quoted for the basic airframe plus a further $983.95 for the engine and related accessories. Extra equipment, not including the radio and armament, gave a grand total of $37,590.45. The total cost for all 320 aircraft, including spares and crating for overseas equipment, came to $14,746,964.35. This was a bargain compared to some of the deals that the BDPC had struck with other manufacturers.

It was from this point that everyone involved in the NA-73X worked extremely hard, conscious of the pressure of the deadline. Accuracy was paramount, with every mathematical and aerodynamic calculation being made without any margin for error. The aircraft, as per Kindelberger and Atwood’s wishes, was designed from the outset for massed production, and every component from the start was made with this purpose in mind. As every component was designed, an accurate wooden mock-up was produced as well, to help production plans and to show that the particular piece of equipment could be installed in the aircraft without difficulty. This also showed, at a very early stage, any snags that might occur before it was fitted into the real airframe.

After 78,000 engineering hours over 127 days, the NA-73X, registered NX19998, was given a ceremonial rollout on Harvard wheels, albeit minus an engine, at Mines Field, Los Angeles, on September 9, 1940. This was 102 days after the Air Ministry contract was signed. The fighter would now sit in Inglewood’s Hangar No.1 until the Allison engine arrived. It was not until October 7 that a single Allison V-1710-39 (F3R) arrived. Within 24 hours, it was installed and the fighter was taxiing under its own power for the first time. Without wasting any more time, the aircraft was first flown from Mines Field on the morning of October 26, 1940 for just 20 minutes by test pilot Vance Breese.

“As clean as a hound’s tooth!” The North American NA-73X, the founding father of the long and successful Mustang line. (NAA)
NA -73X was a beautiful aircraft, every inch a thoroughbred. The list of ground-breaking design features was long, but one of the most significant was the wing. Utilizing information provided by the United States National Advisory Committee for Aeronautics (NACA), the NAA produced a laminar flow wing, which was designed to allow an uninterrupted flow of air over the surface.

Only one flying prototype was ever built, but a static-test airframe, virtually replicating the NA-73X, was also created. The static-test airframe was designated the XX-73 and was used to test the airframe to breaking point, which occurred well above the design load. (NAA)The flight-test program was progressing nicely when Vance Breese, who was a civilian freelance test pilot, handed the NA-73X over to NAA test pilot Paul Balfour. Breese made no secret of his low opinion of Balfour’s abilities, even going as far as to make a bet that he would crash the NA-73X on his first flight. Unfortunately his prophecy proved correct. While on his first familiarization flight, Balfour crashed on approach to Mines Field on November 20, 1940. Part of the flight was to make a high-speed pass over Mines Field to test the NA-73X’s speed between two timing points. After making the first high-speed pass, Balfour forgot to switch fuel tanks. After only being airborne for 12 minutes, the aircraft ran out of fuel and suddenly became a glider. Attempting to turn towards the active runway, Balfour lost height too quickly. With its undercarriage down, the NA-73X landed in a cultivated field and, on touching the soft ground, turned over on its back trapping Balfour in the cockpit. Luckily, the machine did not catchIn a field close to Mines Field lies the undignified wreckage of the once-pristine NA-73X on November 20, 1941. After being airborne for just 12 minutes after takeoff, test pilot Paul Balfour neglected to switch to the correct fuel tank, starving the engine of fuel. (NAA)fire, giving his rescuers time to dig him out. With just 3 hours and 20 minutes flying time under its belt, the aircraft was seriously damaged and would not be back in the air until January 1941. Schmued attempted to brief Balfour on the takeoff and flight-test procedure before Balfour flew, but he refused to listen, saying that “one airplane was like another.”

By the time the NA-73X returned to the air, the flight-test program had been handed over to Robert C. “Bob” Chilton, who would continue to test all variants of the P-51 during World War II. Chilton flew the fighter for the first time on April 3, 1941 and would fly the NA-73X on another 12 occasions. Only one other NAA test pilot is recorded as flying the NA-73X before the aircraft was grounded, namely Louis Wait.

While the NA-73X was being repaired, the RAF had come up with a name for the aircraft that would be adopted universally. Having considered the name “Apache” suggested by NAA, the RAF chose “Mustang.” After the US entered World War II, many pilots flying the A-36 dive-bomber version of the airplane wanted to adopt the name “Apache” to set it apart from the fighter model, which was then known as the P-51. The name was unofficially adopted, but as US forces in western Europe progressed towards Italy the name “Invader” was also suggested for the A-36 when a 27th FBG (Fighter Bomber Group) pilot said “it keeps invading places.” However, despite these unofficial names associated with the A-36, the aircraft was always officially known as a “Mustang.”

The NA-73X continued to operate as part of the NAA’s development program until July 15, 1941, when it was grounded indefinitely. It was destined to be the only prototype of the Mustang. The only other pre-production aircraft was a static-test airframe that is occasionally referred to as the XX-73. This served its purpose by being tested to destruction in January 1941 after the wing structure failed at a point 5 percent higher than the design load.

The ground-breaking prototype spent the remainder of its days at Inglewood before allegedly being donated to a school located near the NAA factory. Whatever the aircraft’s fate, this innovative and beautiful machine eventually met an undignified end as scrap. 

Allison-Engined P-51 Mustang (AVG 1 )Excerpted from:
Allison-Engined P-51 Mustang
Martyn Chorlton
Paperback / $18.95 US / $19.95 Can / £11.99
Air Vanguard (AVG) 1 /9781780961514

For a complete list of available Osprey books on the P-51, including its combat record in WWII and Korea, click here.