The North American P-51 Mustang is a single-engine propeller-driven fighter aircraft flown primarily by American and British forces during World War II. The P-51 has a top speed of 440 mph and was typically armed with six wing-mounted .50 caliber machine guns.
The initial P-51 Mustang design was produced at the request of the British government due to the inferior performance of Britain's fighter aircraft as compared to German models at the onset of war in 1940. The prototype NA-73X Mustang was powered by an Allison in-line engine design that was considered a powerful engine but a poor performer at high altitudes. Early Mustangs were fitted with this engine until 1942, when test pilot Ronald Harker suggested that the plane was well-suited to the Rolls-Royce Merlin 60 engine. The supercharger-equipped Merlin engine and its license-built Packard derivative outperformed the Allison engine and were the power plants for the P-51B Mustang and subsequent versions.
The P-51's high speed and large fuel capacity made it ideal for long-range patrol and bomber escort missions, leading the United States Army Air Force to begin acquiring the fighter in bulk in 1942. Pilots found the Mustang's power to be advantageous in dogfights, though it was not as agile as the Royal Air Force's Spitfires. The combat prowess of the Mustang is considered to be one of the primary advantages leading to the Allied victory in the air war over Europe.