As most Mustang-lovers know, this aircraft was designed and paid for by the British government. Delegates from Great Britain came to the US to acquire combat aircraft (among other things) to stave off the oncoming Nazi threat. While the US was still neutral, its production lines were made available. Having signed a production deal with Curtiss for the P-40, the delegates came to North American Aviation to enlist them to co-produce the P-40. Instead, North American designers convinced the delegates that they'd have a superior aircraft ready to fly in less than a year, and so Mustang I was born. It was powered by the Allison V-1710 engine. While these were great at low level, they could not carry the fight up to medium altitudes due to the limitations of the Allison's mechanical supercharger. The RAF took one of their Mustang Is and hung a Merlin 61 on the nose. They were astonished at the vast improvement in performance of the aircraft in low and medium altitudes, and even able to reach 40,000 feet. North American transitioned production to incorporate the license-built Packard Merlin V-1650 engine, and the P-51B/C were born.