Initially produced as a response to a 1940 RAF requirement for a fast, heavily armed fighter able to operate effectively at altitudes in excess of 6100m (20,000ft), the P-51 Mustang went on to become one of the most famous fighters of WWII. The first of 320 production Mustang Is for the RAF flew on 1 May 1940, powered by a 1100hp Allison V-1710-39 engine. Trials with Packard built Rolls-Royce Merlin 61 engines showed a dramatic increase in performance, maximum speed being raised to 441mph (710kmh), an increase of about 50mph from the Allison engined varient. Production of the Merlin powerrd P-51B got underway in late 1942. The first mission of the P-51B was the 354th Fighter Group, who flew an escort mission from England in December 1943. The P-51D was the most numerous varient, a production totalling 7956. In 1944, they operated of the captured islands of Iwo Jima and Okinawa and escorted B-29s to their targets. This P-51C bears the distinctive red tail of 332nd fighter group. They were based in italy.