The unpainted North American NA-73X prototype NX19998, which became the P-51 Mustang, parked on the ground in 1940

NA-73X Prototype: The 1940 Plane That Became the P-51 Mustang

by Giuseppe Duncan | Oct 26, 2026 | Storia e leggende, Aviazione militare | 0 comments

On the morning of 26 October 1940, a bare-metal fighter with no paint and not even a name sat waiting at Mines Field, the small Los Angeles airfield that would one day become LAX. Freelance test pilot Vance Breese climbed in, started the Allison V-12 and took it up for its first flight.

The aircraft was the NA-73X, and North American Aviation had never built a fighter before. Barely five months earlier, the British had only just signed the order for it. Within four years its descendants would be escorting bombers all the way to Berlin as the P-51 Mustang.

Eighty-six years later, the story of how it was built still reads like a dare. The British asked for P-40s. North American said no, and offered something better.

Informazioni rapide

Aeromobili: North American NA-73X, prototype of the Mustang (registration NX19998)
Ordinato da: British Purchasing Commission, 1940
Designer: Edgar Schmued’s team at North American Aviation
Airframe complete: 9 September 1940, 102 or 117 days depending on the starting date used
Primo volo: 26 October 1940, Mines Field, Los Angeles (the Smithsonian gives 25 October)
Pilota: Vance Breese
Motore: Allison V-1710 V-12, single-stage supercharger
Incidente: 20 November 1940, fuel starvation; pilot Paul Balfour unhurt

The order nobody wanted to fill

In 1940 Britain was desperate for fighters, and the British Purchasing Commission in the United States, headed by Sir Henry Self, was shopping for anything that could fight. The only American design that came close to European standards was the Curtiss P-40, and Curtiss’s factory was running flat out.

North American was already selling the T-6 Texan trainer to the Royal Air Force as the Harvard. When company president James “Dutch” Kindelberger came looking for more business, Self asked whether North American would build P-40s under licence. Kindelberger replied that his company could get a better fighter, with the same Allison engine, into the air sooner than it could set up a P-40 production line.

“Well, Dutch, don’t let us build an obsolete airplane, let’s build a new one. We can design and build a better one.”

Edgar Schmued — Lead designer of the NA-73X, answering Kindelberger’s question whether North American should build P-40s

It was a bold claim from a firm with no fighter experience. The British set hard terms: four machine guns, a unit price of no more than $40,000, and, according to several histories, a prototype within 120 days. North American bought Curtiss’s P-40 wind-tunnel and test data, and on 29 May 1940 the commission firmly ordered 320 aircraft. A design team led by lead engineer Edgar Schmued got to work.

102 days, or 117?

Schmued’s team worked at a pace that became legend. The airframe of the NA-73X was complete on 9 September 1940, still waiting for its Allison engine. Exactly how fast that was depends on where you start the clock, and the sources do not agree.

Wikipedia, aviation historian Joe Baugher and The National WWII Museum all give 102 days, counted from the British order. The Smithsonian National Air and Space Museum and This Day in Aviation say 117 days, counted from when the prototype was authorised. Even the first-flight date wobbles: the Smithsonian gives 25 October, while Boeing, which now owns North American’s heritage, and most other sources give 26 October.

Either way, it beat the 120-day target. Counting to the first flight, Wikipedia puts it at 149 days into the contract, an extraordinarily short development time even in wartime. According to This Day in Aviation, that first flight lasted just five minutes, and Breese took the aircraft up for another ten later the same day.

The second North American XP-51 Mustang parked at NACA Langley, the first aircraft to use an NACA laminar-flow airfoil — The second XP-51, one of two early Mustangs handed to the US Army for evaluation, at NACA’s Langley laboratory in 1943. The Mustang was the first aircraft to use an NACA laminar-flow airfoil. NASA photo GPN-2000-001247 / public domain

Clean lines and a new wing

The speed was not just about long shifts. The NA-73X was designed from the start for mass production, built in five main sections that were fitted out with wiring and piping before being joined together. Its fuselage was lofted mathematically, giving smooth, low-drag curves.

The NA-73X prototype had two features that made it fast. The first was a laminar-flow wing, developed with the National Advisory Committee for Aeronautics, designed to keep the airflow smooth over more of its surface. The second was the radiator, moved behind the cockpit into a belly scoop, where heated air leaving the duct even produced a little thrust.

A mechanic works on the exposed Allison V-12 engine of an RAF North American Mustang Mk I on an airfield in Buckinghamshire — A mechanic adjusts the Allison V-1710 of an RAF Mustang Mk I during Exercise Spartan in Buckinghamshire. The same engine powered the NA-73X and the P-40. War Office official photograph H 27819, Imperial War Museums / public domain

In Breese’s hands the prototype handled well and carried an impressive load of fuel. That fuel capacity, almost a footnote in 1940, would later decide the air war over Germany.

Archival North American Aviation film of a Mustang factory test flight, from Boeing’s heritage channel, Boeing Classics.

Out of fuel on flight day

On 20 November 1940, North American’s chief test pilot Paul Balfour took over for a high-speed run. According to This Day in Aviation, Schmued offered to walk him through the cockpit routine first. Balfour waved it away, saying “one airplane is like another.”

“Before this flight, I asked Balfour to get into the airplane and go through the routine of a takeoff and flight. He responded that one airplane is like another and he would not need the routine checkout.”

Edgar Schmued — Designer of the NA-73X, on the morning of the 20 November 1940 crash (via This Day in Aviation)

About twelve minutes into the flight, the Allison stopped. Balfour had forgotten to switch fuel tanks. Too far from the runway, he put the prototype down in a freshly ploughed field, where the wheels dug in and the aircraft flipped onto its back.

Balfour was not hurt and crawled out of the upside-down wreck. Most histories say the prototype was rebuilt and flying again in 1941, with Robert Chilton taking over the test programme, though one account holds that it never flew again. Either way, the design had already proved itself.

A low-level thoroughbred

The US Army Air Corps had played no part in the deal, but two early production aircraft were handed over for evaluation as XP-51s. The British, meanwhile, named their new fighter the Mustang.

The Mustang I reached RAF units in early 1942 and flew with Army Co-operation Command on low-level tactical reconnaissance and ground attack. Down low it was about 30 mph faster than the P-40, even with the same Allison engine, and faster than a Spitfire V at low and medium altitude.

An early Allison-engined North American Mustang Mk I in RAF roundels flying over English farmland, photographed from another aircraft — An early Allison-engined RAF Mustang in flight, circa 1942. Fast and long-legged at low level, the Mustang I served with Army Co-operation Command. RAF official photograph CH 6710, Imperial War Museums / public domain

The problem was altitude. The Allison had only a single-stage supercharger, and above about 15,000 feet the Mustang’s performance fell away fast. That was exactly where the air war over Europe was being fought.

British Movietone footage of Allison-engined RAF Mustang Mk Is in 1942, via AP.

Enter the Merlin

In spring 1942 Rolls-Royce test pilot Ronald Harker flew a Mustang I and came back with an idea: fit it with the two-stage supercharged Merlin from the Spitfire IX. Rolls-Royce converted airframes at Hucknall, and the Merlin-powered Mustang X flew in October 1942.

In the United States, North American built its own Merlin prototype, the XP-51B, which flew in November 1942 with the Packard-built version of the engine. The US Army Air Forces was so impressed that it had already ordered 400 three months earlier.

“Sired by the English out of an American mother, the Mustang has no parent in the Army Air Corps to appreciate and push its good points.”

Major Thomas Hitchcock — US Assistant Air Attaché in England and early champion of the Merlin Mustang

The rest is legend. The P-51 Mustang went on to escort American bombers deep into Germany, and more than 15,000 were built. If you want to see how the finished article stacks up against the best of the Spitfires, read our Spitfire XIV vs P-51D comparison.

It all started with one unpainted prototype, a five-figure price cap and a company that had never built a fighter. North American did not build the P-40s the British asked for. It built the aircraft that replaced them.

Greg’s Airplanes and Automobiles explains where North American Aviation came from, and how it could turn out the T-6, B-25 and P-51.

Sources: Wikipedia (North American P-51 Mustang; Rolls-Royce Mustang Mk.X; Edgar Schmued; Vance Breese), Joe Baugher, Smithsonian National Air and Space Museum, The National WWII Museum, This Day in Aviation (Bryan R. Swopes), Boeing Classics

Domande frequenti

What was the NA-73X?

The NA-73X was the prototype of the North American P-51 Mustang. North American Aviation designed it in 1940 for the British Purchasing Commission, which had asked the company to build Curtiss P-40s. The unpainted prototype, registered NX19998, was powered by an Allison V-1710 engine and first flew at Mines Field in Los Angeles.

When did the NA-73X first fly?

The NA-73X first flew on 26 October 1940 at Mines Field, Los Angeles, now Los Angeles International Airport, with freelance test pilot Vance Breese at the controls. Most sources, including Boeing and The National WWII Museum, give 26 October, although the Smithsonian National Air and Space Museum lists the date as 25 October 1940.

How long did it take to build the NA-73X?

The NA-73X airframe was completed on 9 September 1940. Wikipedia, Joe Baugher and The National WWII Museum say that was 102 days after the British order, while the Smithsonian and This Day in Aviation say 117 days after the prototype was authorised. Either figure beat the British target of 120 days.

What happened to the NA-73X prototype?

The NA-73X crashed on 20 November 1940 when test pilot Paul Balfour forgot to switch fuel tanks and the engine stopped. He landed in a ploughed field, where the aircraft flipped over, but he was not hurt. Most histories say the prototype was rebuilt and flew again in 1941, though one account says it never flew again.

Who designed the P-51 Mustang?

The P-51 Mustang was designed by a North American Aviation team led by lead engineer Edgar Schmued, under company president James “Dutch” Kindelberger. Schmued’s team designed the NA-73X prototype in 1940 for the British Purchasing Commission, with a laminar-flow wing developed with NACA and a low-drag radiator installation behind the cockpit.

Why was the Mustang given a Merlin engine?

The early Mustang used an Allison V-1710 engine with a single-stage supercharger, so its performance fell off above about 15,000 feet. In 1942, Rolls-Royce test pilot Ronald Harker proposed fitting the two-stage Merlin from the Spitfire IX. The Merlin transformed high-altitude performance and led to the P-51B, built with the Packard-made version of the engine.