The T7 light tank series had several experimental design configurations. Two of them the T7 and the T7E1 were not constructed, whereas prototypes were built for the T7E2, T7E3, and T7E4.
The T7E2 was selected as the most promising but it was upgunned from an initial 37 mm to a 57 mm gun. In June 1942 this was finished and then the Armed Forces asked for it to be upgunned further to a 75 mm gun. Once this was done it brought the tank up in weight from a light tank to a medium tank it became the M7 Medium Tank.