In 1940 the British approached North American Aviation to license-build Curtiss P-40 fighters for the Royal Air Force. Reluctant to build a competitor’s aircraft, North American offered to build a new aircraft using the same American-built Allison engine as the P-40. The British agreed, but only with the unreasonable stipulation that the first aircraft had to be flying in no more than 120 days. A remarkably short 117 days later, on October 1940, the first prototype flew as NA-73X. Production of the aircraft — named Mustang I by the British — began the following year.

Although a great low altitude airplane, the Allison-powered Mustang could not compete at high altitude with the Spitfires powered by the British Rolls Royce Merlin engine. Because it was equipped with a more sophisticated supercharger, the Merlin could maintain its rated power to a much higher altitude than the Allison. In the fall of 1942, Mustangs in the United States and Great Britain were experimentally fitted with British Merlin engines and a legend was born. Almost 100 mph faster at altitude than its Allison-powered counterparts, the Merlin-powered Mustang with its laminar flow wing was to become one of best and most well-known fighters of World War II. Possessing excellent range and maneuverability, the P-51 operated primarily as a long-range escort fighter, but also as a ground attack fighter-bomber. The Mustang served in nearly every combat zone during WWII, and later fought in the Korean War.