Despite the launch of mass production of the M3 medium tank in the summer of 1941, the development of an improved version with a 75 mm gun in a rotating turret was started earlier in winter of the same year. Two prototypes were produced by the autumn of 1941. The military was mostly satisfied with the improved version but also requested some improvements. As a result, the vehicle lost its side doors and the commander's cupola machine gun. On September 5, 1941, the Armaments committee recommended standardizing the vehicle under the Medium Tank M4 designation.