The second of the three MB.3 prototypes ordered by MAP, R2496, was intended to be Griffon-powered, and was referred to as the MB.4. At the time of R2492's crash, the necessary drawing amendments for the MB.4 were complete and manufacture had started.
It was intended to enter the competition against Specification F6/42 but the attempt was unsuccessful. However, Martin felt that he could improve on the MB.3/MB.4 concepts. R2496 was left uncompleted and work on the third intended aircraft, R2500, was not begun. Early in 1943, the difficult decision was taken to start afresh, tempered by the knowledge that the firm had received verbal assurances from MAP that if a new fighter design were produced, a contract for one prototype would be awarded, again to Specification F5/39.
With the then hand-built nature of a prototype, in an experimental shop, with design adjustments made on the spot, and drawings brought into line afterwards, made resources available out of all proportion to the effort. By 1943, when work on the MB.5 had begun.