The M.35 was completed and flown in six weeks but proved to be unstable. Wind tunnel tests showed the problems to be curable and George Miles conceived a heavy bomber, the M.39, to be powered by three turbojet engines, or in its initial form with two high-altitude Rolls-Royce Merlin 60s or Bristol Hercules VIIIs.

A five-eighths scale model of the bomber was built and designated M.39B, flying for the first time on 22 July 1943 and proving to be aerodynamically stable. Flight trials were initially on a private basis, but in 1944 the M.39B went to the Royal Aircraft Establishment at Farnborough, where it suffered two accidents. After extensive repairs the work ended.