The early P-51s were built on "soft" tooling, slow and inefficient. USAAF wanted to upgrade NAA's tooling but had no budget for it. Congress found money in the "marching Army" budget, but couldn't spend it on a fighter. Thus NAA drew a couple more lines on the plans, called it an A-36 ground attack plane, and won a contract for 500. Of course, that funded the new tooling set, making mass production of the P-51 possible
The NAA held orders from the British for 770 aircraft (620 paid for by the Brits plus 150 to be supplied under lend lease) prior to the USAAF placing any orders. The Pursuit Board held meetings between 11 and 30 October 1941 and produced a document "Future Development of Pursuit Aircraft" which examined 18 experimental and 8 production aircraft - the Mustang was not among them.
With the reputation of the Stuka in the background, the USAAF planned on using the Vengeance in the dive bombing role. On 4 February 1942, Col. K. B. Wolfe, Chief of the Production Engineering Section wrote, "We will not have a useful dive bomber before March 1943". He recommended cancelling the Vengeance and obtain "a suitable dive bomber, low altitude attack fighter in its place".
NAA was approached and work began on the project 16 April 1942 and testing of the A-36 began 30 May. The USAAF issued contract AC-27396 on 21 August for 500 A-36.
NAA used "soft" tooling throughout, as it enabled them to make rapid modifications, and was a feature of all NAA production at the time. The Harvard NAA built had 2,500 modifications over 25 variants of the first 1,000 aircraft for example.