 The project was canceled after U.S. Marines secured the Japanese island of Iwo Jima and its airfields, making it possible for standard P-51D models to accompany B-29s all the way to the Japanese home islands and back.While North American were concentrating on improving the performance of the P-51 through the development of the lightweight Mustangs, in Britain, other avenues of development were being pursued.To this end, two Mustang Mk IIIs (P-51Bs and P-51Cs), "FX858" and "FX901", were fitted with different Merlin engine variants.