 The RAF named both these models Mustang Mk.III.In performance tests, the P-51B achieved 441 mph (710 km/h) at 25,000 ft (7,620 m), and subsequent extended range with the use of drop tanks enabled the Merlin-powered Mustang version to be introduced as a bomber escort.In June 1943, Rolls-Royce proposed to re-engine the Mustang with a Griffon 65, although the resultant "Flying Test Bed" (F.T.B.)