The P-51D, which was the premiere US fighter aircraft of WWII, was originally developed in the remarkable time of 117 days in response to a request from the British Purchasing Commission. The first Mustangs were powered by Allison engines and had limited high altitude performance. But when the Allison was replaced with the Rolls-Royce Merlin the P-51 evolved into a superb long-range bomber escort. The definitive D-model is identified by its bubble canopy. P-51Ds remained in US service until the early 1950s.