In 1940 British purchasing agents asked North American Aviation to build more Curtiss P-40s. The P-40 was the only American land-based fighter available at the time but it was also seriously deficient in speed, range, and altitude performance. North American declined to build it but proposed a completely original design and promised to finish it in less than three months. North American completed the NA-73 airframe 117 days later and installed an in-line, liquid-cooled Allison engine. This aircraft first flew on October 25, 1940. During flight tests, the airplane exhibited outstanding performance, particularly in level-flight speed. The British enthusiastically ordered 150 examples of the airplane and the Royal Air Force (RAF) named it the Mustang. The U. S. Army Air Corps also showed interest and in 1941 they ordered 500 ground-attack variants designated the A-36.
Early in 1942, the British tested four RAF Mustangs fitted with the two-stage, two-speed Merlin 65 engine. The supercharger in the Merlin 65 was optimized to produce sea-level horsepower up to approximately 9,150 m (30,000 ft). Urged on by Rolls Royce and British and American fighter pilots who sampled the new Mustang, North American obtained two Merlin 61 engines similar to the 65 and installed them in airframes designated XP-51Bs. The new variant had good range and outstanding speed, about 80 kph (50 mph) faster than previous models. Thus was born the world's best long-range, propeller-driven escort fighter. With improved range and speed, P-51 squadrons could now accompany 8th Air Force bombers on long-range raids over Europe. Bomber losses dropped sharply when fighters could protect them during their entire mission. The Mustang also proved itself in the Pacific theater. Again, the P-51 had the range, speed, and endurance to escort Boeing B-29 Superfortresses conducting long-range bombing attacks. The newly independent U. S. Air Force used the airplane during the Korean War for close-support, ground attack missions but it was not suited for this dangerous mission. One of the Mustang's few vulnerable spots was the cooling system. A single bullet through a radiator or pipe was usually enough to down a P-51.
North American built more than 14,000 Mustangs and more D-models (8,302) than all other variants combined. The most significant D-model features were a rear fuselage reduced in height to accommodate a new bubble canopy and an increase in armament from 4 to 6 fifty-caliber machine guns. The Air Force did not withdraw P-51s from service until 1957.
North American built the NASM P-51D-30-NA, Air Force serial number 44-74939, late in 1945. It was not delivered until July 1945 and never saw combat. The Army Air Forces first assigned the fighter to Andrews Field, outside Washington, D.C., and later to Freeman Field, Indiana. After eleven months and 211 flying hours of operational service, this Mustang was set aside as a museum specimen and later transferred to the National Air Museum. When NASM acquired this Mustang, it bore a proud admonition on both sides of the fuselage in large black letters: "Guard The Victory, Join the AAF." The airplane was probably used for recruiting drives toward the end of its military career. For exhibit purposes, this Mustang is painted in the colors and markings of the 351st Fighter Squadron, 353rd Fighter Group, 8th Air Force. This unit converted from North American P-47 Thunderbolts to Mustangs on September 30, 1944, while stationed at Raydon, Suffolk, England. The 353rd is typical of the many units assigned to escort bombers on missions deep into Germany. After air engagements to protect the bombers, the P-51s of the 353rd would attack enemy aircraft and ground installations on strafing missions on their way home. This fighter group claimed 330 1/2 aircraft shot down and 414 destroyed on the ground. It was awarded the Distinguished Unit Citation for the support of airborne landings in Holland.