On September 9, 1940, only 102 days after the contract had been signed, the NA-73X prototype was rolled out though still waiting for its engine. The new
fighter was named Mustang by the British, and the first British version was designated Mustang I. As soon as it was available, the 1,120hp Allison V-171 0-39
powerplant was installed, and engine and taxi tests began. On 26 October, Vance Breese lifted the aircraft off the runway for a maiden flight. Testing
continued until Paul Balfour was forced to make a deadstick landing. The NA73X flipped over on its back, and it took six weeks to make repairs and get the
aircraft ready to fly again. The first production Mustang I soon joined the repaired prototype in the test program, and shortly other Mustangs were heading for
As expected, the evaluation of the flight test aircraft showed that the NA-73 was indeed superior to the P-40 Warhawk which was considered to be the best
single-engine fighter in the U.S. Army Air Corps inventory at the time. A lot of criticism has been written about the AAC's initial lack of interest and
subsequent slowness to act concerning acquiring Mustangs for its own use. This simply is not so. All evidence is to the contrary, particularly considering the
political and economic situation in America at that time. Here again Lt. Benjamin Kelsey was the man who deserves the credit.
Kelsey had seen to it that two aircraft from the original British order were supplied to Wright Field for testing, but his efforts did not stop there.
Kelsey was able to place an order for 150 P-51s on July 7, 1941, and this was before the first XP-51 even arrived in Ohio. It was a small order for the first
U.S. version, but it was all that funding would allow at that time.
The P-51 was very similar to the Mustang I, with the main difference being the replacement of the mixed machine gun armament of the British version with
four 20mm cannon. Clearly, the USAAC had a genuine interest in the NA-73, even before it had a chance to obtain one. The situation in Europe would cause 93 of
the P51s to be sent to the Royal Air Force where they were called Mustang IAs.
With an order from the U.S. Army, North American chose the nickname "Apache" for the U.S. aircraft. But by then the British name, Mustang, had taken root,
and it became the official nickname of the USAAC.
With no funds available for pursuit aircraft, as fighters were then called, Lt. Kelsey came up with a way to beat the system and get some additional
Mustangs on order for the USAAC. Using some remaining funds for attack aircraft, Kelsey asked North American to develop a dive-bomber version of the NA-73.
Choosing A-36, which was the next available attack designation, Kelsey ordered 500 of these dive-bomber versions on April 16, 1942. Ironically, the North American
name, "Apache," was commonly used with the A-36 but never officially recognized by the newly renamed U.S. Army Air Forces.
About a month and a half after the order for 150 P-51s had been made on July 7, 1941, the first XP-51 arrived at Wright Field on August 24.
The second did not join the first until December 16. In between these deliveries, the Japanese had caused everything in the United States to change with their
attack at Pearl Harbor.
In numerous accounts about the development of the Allison-powered Mustang, it has been stated that its poor performance at high altitudes was a surprise and
a disappointment to the British and to the USAAF as well. This simply is not so. The aircraft designers of that day had more than sufficient knowledge of
powerplants, and they were capable of determining that the Allison engine and supercharger combination installed in the aircraft would have a drop in
performance above 15,000 feet. Clear evidence of this is that two of the P-51s, ordered even before the flight of the first XP-51, were reserved for testing
with a Packard-built Rolls Royce Merlin engine. They knew that the Allison-powered Mustangs would be low-Ievel fighters, while the Merlin-powered aircraft
would be the high-altitude versions.
By comparison, Mustangs with the Allison engine could outperform the Merlin-powered variants below 15,000 feet, but no writer has criticized the P-51B,
-C, -D, or -K for having less performance at low altitudes. It has been claimed that the Merlin engine is what allowed the Mustang to reach its potential.
If only high-altitude performance is considered, this would be true. But a more correct assessment would be that both the Allison and Merlin-powered versions
performed very well at the altitudes where they were designed and intended to operate.