Ahhhh when life was simpler: The British Purchasing Commission stipulated to North American an armament of four .303 in machine guns, the Allison V-1710 liquid-cooled engine, a unit cost of no more than $40,000, and delivery of the first production aircraft by January 1941 The prototype NA-73X was rolled out in September 1940, just 102 days after the order had been placed; it first flew on 26 October 1940, 149 days into the contract. The first Mustang Mk Is entered RAF service in January 1942, the first unit being 26 squadron. Yes it needed to be re-engined – but it was operational.
And that included all the wind tunnel testing of two possible aerofoils resulting in the new laminar-flow wings. It was a mathematically designed aircraft and the wing foil that was to be classified as a "semi-empirical venture" by the British was cleared for adoption on the new design.. So for its day it was high-tech.