The North American Aviation P51 Mustang was an American long-range single seat fighter aircraft that entered service with Allied air forces in the middle years of World War II. The P51 became one of the conflict's most successful and recognisable aircraft.

The P51 flew most of its wartime missions as a bomber escort in raids over Germany, helping ensure Allied air superiority from early 1944. It also saw limited service against the Japanese in the Pacific War. The Mustang began the Korean War as the United Nations' main fighter, but was relegated to a ground attack role when superseded by jet fighters early in the conflict. Nevertheless, it remained in service with some air forces until the early 1980s.

As well as being economical to produce, the Mustang was a fast, well made and highly durable aircraft. The definitive version of the single-seat fighter was powered by the Packard V16503, a two stage two speed supercharged 12 cylinder Packard built version of the legendary Rolls Royce Merlin engine, and armed with six of the aircraft version of the .50 caliber (12.7 mm) Browning machine guns.

After World War II and the Korean conflict, many Mustangs were converted for civilian use, especially air racing. The Mustang's reputation was such that, in the mid 1960s, Ford Motor Company's Designer John Najjar proposed the name for a new youth oriented coupé after the fighter.

In 1939, shortly after World War II began, the British government established a purchasing commission in the United States, headed by Sir Henry Self. Along with Sir Wilfrid Freeman, who, as the "Air Member for Development and Production", was given overall responsibility for RAF production and research and development in 1938. Self had sat on the (British) Air Council Sub committee on Supply (or "Supply Committee"), and one of Self's many tasks was to organise the manufacture of American fighter aircraft for the RAF. At the time the choice was very limited: none of the U.S. aircraft already flying met European standards; only the Curtiss P40 Tomahawk came close. The Curtiss plant was running at capacity, so even that aircraft was in short supply.

North American Aviation (NAA) was already supplying their Harvard trainer to the RAF, but was otherwise underutilised. NAA President "Dutch" Kindelberger approached self to sell a new medium bomber, the B-25 Mitchell. Instead, self asked if NAA could manufacture the Tomahawk under licence from Curtiss.

Kindelberger replied that NAA could have a better aircraft with the same engine in the air in less time than it would take to set up a production line for the P40. As executive head of the British Ministry of Aircraft Production (MAP), Freeman ordered 320 aircraft in March 1940. On 26 June 1940, MAP awarded a contract to Packard to build modified versions of the Rolls Royce Merlin engines under licence; in September, MAP increased the first production order by 300.

The result of the MAP order was the NA73X project (from March 1940). The design followed the best conventional practice of the era, but included two new features. One was a new NACA designed laminar flow wing, which was associated with very low drag at high speeds. Another was the use of a new radiator design (one Curtiss had been unable to make work) that used the heated air exiting the radiator as a form of jet thrust in what is referred to as the "Meredith Effect". Because North American lacked a suitable wind tunnel, it used the GALCIT 10 foot wind tunnel at Cal Tech. This led to some controversy over whether the Mustang's cooling system aerodynamics were developed by North American's engineer Edgar Schmued or by Curtiss, although historians and researchers dismiss the allegation of stolen technology; such claims are likely moot in any event, as North American had purchased Curtiss' complete set of P40 and XP46 wind tunnel data and flight test reports for US$56,000.

While the United States Army Air Corps could block any sales it considered detrimental or not in the interest of the United States, the NA73 represented a special case. In order to ensure deliveries were uninterrupted, an arrangement was eventually reached where the RAF would get its aircraft in exchange for NAA providing two free examples to the USAAC for evaluation.

The prototype NA73X was rolled out just 117 days after the order was placed, and first flew on 26 October 1940, just 178 days after the order had been placed, an incredibly short gestation period. In general, the prototype handled well and the internal arrangement allowed for an impressive fuel load. It was armed with four .50 M2 Browning (12.7 mm) guns located in the wings, and two .30 caliber Browning (7.62 mm) guns in the cowl.

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