North American P-51 Mustang Mustang Mk X (Rolls-Royce conversion)

In April 1942 the RAF's Air Fighting Development Unit found the Allison-engined Mustang Mk I lacking at higher altitude. Rolls-Royce test pilot Ronnie Harker flew the aircraft, and company engineers concluded a high-altitude Merlin with a two-stage supercharger would transform its performance. Rolls-Royce converted five Mustang Mk I airframes with the Merlin 65 (a Merlin 61, as fitted to the Spitfire Mk IX, was considered but never installed on a Mustang) under the designation Mustang Mk X, keeping the Mk I airframe and radiator duct, with a four-bladed 3.28 m Rotol propeller taken from a Spitfire Mk IX. Results were remarkable: aircraft AM208 reached 697 km/h at 6,700 m with full supercharger boost, and AL975 was tested to an absolute ceiling of 12,400 m. Two of the five prototypes were handed to the U.S. Eighth Air Force in Britain for evaluation. The conversion's success paralleled development of the more thoroughly engineered XP-51B with the Packard V-1650-3, which made a planned production run of 500 Mk X aircraft unnecessary.