One of the most fascinating aspects of the North American P-51 Mustang's development is how rapidly and unexpectedly it was conceived. The Mustang, later hailed as one of the finest fighter aircraft of World War II, was initially born not out of American ambition, but British desperation. Even more astonishing, the aircraft’s prototype was designed, built, and ready to fly in just 117 days—a remarkable engineering feat by any standard.
In early 1940, the British Purchasing Commission approached North American Aviation (NAA) to build more Curtiss P-40 Warhawks under license. Instead of producing an older design, NAA president James “Dutch” Kindelberger boldly proposed something better: a brand-new, modern fighter tailored to British needs. The British, intrigued but skeptical, gave the go-ahead—but on one condition. NAA had to deliver the first prototype in just 120 days.
The result was the NA-73X prototype, the foundation of what would become the P-51 Mustang. With a small team of engineers and around-the-clock shifts, NAA completed the prototype in only 117 days. The aircraft first flew on October 26, 1940. This rapid development was not only unprecedented but also resulted in an airframe that was ahead of its time—featuring laminar flow wings, a sleek fuselage, and a fully retractable landing gear. The Mustang’s initial design was efficient, fast, and aerodynamically advanced.
However, the early Mustangs were powered by the Allison V-1710 engine, which performed well at low to medium altitudes but suffered above 15,000 feet. This limitation made it unsuitable as a high-altitude interceptor or bomber escort, roles that would become critical as the air war intensified over Europe. Despite this, the British initially used the Mustang for tactical reconnaissance and low-level ground attack roles, where it excelled.
The real turning point came when Rolls-Royce engineers suggested fitting the Mustang with the legendary Merlin engine—the same powerplant used in the Spitfire and Hurricane. When a Merlin-powered Mustang prototype (designated XP-51B) was test-flown, it demonstrated outstanding high-altitude performance, combining the aerodynamic efficiency of the Mustang with the power and altitude capability of the Merlin. The U.S. quickly adopted this configuration, and the definitive P-51B/C and later P-51D variants were born.
With drop tanks added to extend its range, the P-51 Mustang became the long-range escort fighter the Allies desperately needed. It could accompany B-17 and B-24 bombers deep into German territory and engage Luftwaffe fighters on equal or superior terms. The introduction of the Merlin-powered Mustang dramatically improved bomber survival rates and contributed significantly to Allied air superiority over Europe.