From Joe Baugher:
"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.
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, the manager of NAA James H. "Dutch" 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, who was an excellent businessman as well as aeronautical engineer, 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."
To continue:
"The NA-73X prototype contract was signed on May 23, 1940. The British insisted that a heavy eight-gun armament be fitted, 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.
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."
Now we fast forward to the finished product:
The NA-73X (Mustang I) was produced in limited numbers and most were held back for evaluation and photo recon roles, the first arriving at Liverpool on 24 October 1941. The NA-83 were a slightly modified version of the Mustang I and the British ordered 300 of them in December of 1940.
On 11 March 1941, the lend/lease act was passed, and the next batch of Mustangs, designated NA-91 (Mustang IA) had the MGs traded for 4 Hispano 20mm cannon in the wings.
At this point, the British had been able to actively employ their Mustangs in combat and came to a conclusion on it's performance.
From Joe Baugher:
"The British decided that the relatively poor high altitude performance of the Mustang was more than just a minor deficiency, since most aerial combat over Europe at that time was taking place at medium to high altitudes. Consequently, the Mustang I was used for low-level tactical reconnaissance and ground attack, where full advantage could be taken of its exceptional low-altitude performance."
Now, let's fast forward to 16 April 1942, when the U.S. Army Air Force placed an order for 500 NA-97, specifically designed for ground attack and designated the A-36. The RAF was given one in March 1943 for evaluation (EW998).
August 1942 saw a USAAF order for 1,200 NA-99 (P-51A) which was the start of the Mustang's legacy with the USAAF.
The British didn't ask for a low level ground support aircraft, they were looking for a fighter. They discovered that their new fighter was not suitable for the type of air warfare that had evolved in the skies over Britain between the time they ordered it and the time they were able to put it to work. They did, however, exploit it's strengths which were "its exceptional low-altitude performance".