The use of multiple engines, suspensions, and transmissions on one type of tank was a unique feature of American tank building. The same thing happened with the prospective replacement of the Medium Tank M4. Development of three types of transmission and several types of suspension theoretically allowed the military to choose the most reliable variant that would then enter production. The result was unexpected. The Medium Tank T23 was chosen as a result of trials, but it never replaced the M4, although some of its elements came in handy.
Work on the tank that would replace the Medium Tank M4 began in the spring of 1942. The Medium Tank T20 project was approved by the Ordnance Department and Ordnance Committee by the end of May. The full sized model was approved around then. The tank remained in the same weight category as the M4, but the firepower, armour, and mobility were improved. The transmission was moved to the rear of the hull and the gearbox, differential, and engine were combined into one unit, which made removal and installation easier. The tank used a reworked Ford GAA engine, a planetary gearbox, and a torque converter.