Martin was devastated by the loss of his partner, but he was a determined man. The Air Ministry had drawn up Specification F.18/39 for the MB3, which was intended as a potential successor to the Hurricane and Spitfire in the Royal Air Force. The engine preferred by Martin had been the nascent Rolls-Royce Griffon, and it was planned to install this in the second of the three MB-3s ordered. Unfortunately it was not available in time, and the ministry selected the much heavier, Sabre as an alternative, much to Martin's disapproval.

There was much haggling after the tragedy. The upshot was that, due to Martin's persistent urging, the second and third machines were redesigned to have the Griffon, as well as incorporating many other changes that Martin regarded as essential, and redesignated MB5, (The MB4 was an unbuilt Bristol Centaurus-engined variant). In addition to an intended high top speed of at least 400mph, the MB3 had been designed for exceptional serviceability, and this feature was retained. Late in its short life the MB3 had been fitted with a bubble canopy. Martin kept this, and further improved the pilot's view by moving the cockpit forward some five feet.

The trouble was that James Martin, the aircraft's sole designer, was a perfectionist, and the MB5's design and construction became a protracted process. The third aircraft was eventually cancelled, and the delivery date of the second slipped repeatedly. It was initially due to be handed over on January 1, 1943, but it did not go to the Armament Experimental Establishment (A&AEE), at Boscombe Down for official trials until late February 1946.

In fact, the MB5 made its maiden flight long before that, at RAF Harwell on May 23, 1944. The pilot, Bryan Greensted, Chief Test Pilot for airscrew manufacturer Rotol, found it directionally unstable, and it was not flown again until several months of lamentably slow work on the rear fuselage and tail services had remedied this. Nonetheless the MB5's outstandingly fine structure and accessibility won the approval of Sir Stafford Cripps of the Ministry of Aircraft Production



