The Mustang is widely regarded as the most successful fighter aircraft of the Second World War. Developed to meet an urgent British request for fighter aircraft, the prototype first flew on 26 October 1940. It was rushed into production and entered service with the Royal Air Force (RAF) in October 1941, but poor performance saw it relegated to a low-level ground-support role. In an attempt to solve the Mustang's performance problems, prototypes fitted with Rolls-Royce Merlin engines were developed in late 1942. The result was an aircraft of superior, long-range, high-altitude performance that was ordered in large numbers by the RAF (as the Mustang III) and the USAAF (as the P-51B and P-51C); these began entering service in June 1943. Further design refinements in late 1943 resulted in the Mustang IV/P-51D and P-51K, which featured a distinctive "teardrop" canopy. 3 Squadron RAAF began operating a mixture of Mustang IIIs and IVs in Italy in November 1944, and 450 Squadron (its sister squadron) was in the process of re-equipping with Mustang IVs when the war ended. In the meantime, a decision had been taken to procure 690 Mustangs for the RAAF in the South-West Pacific, the bulk of which would be locally produced. Ultimately, 298 US-built P-51D and P-51K models were acquired and 200 P-51Ds were built by the Commonwealth Aircraft Corporation as the CA-17 and CA-18; Australian production ceased in August 1951. Overall, 15,576 Mustangs were produced across the world. The Second World War ended before the RAAF could employ the Mustang operationally, but it was used extensively by 77 Squadron RAAF in a ground-attack role in the early months of the Korean War. After serving with 15 RAAF squadrons - 3, 4, 21, 22, 23, 24, 25, 75, 76, 77, 78, 82, 84, 86, and 450 - the last Australian Mustangs were retired in June 1960.