On August 29, 1940, (the day after the M3 was decided to be put into production) work began on a tank that would mount a 75 mm gun in the turret. First prototype (T6) was built at the Aberdeen Proving Ground and appeared in September 1941. Production started at 1,000 per month and increased to 2,000 and was built at 11 plants. Production was able to quickly switch over from the M3 to the M4 as they both shared bottom hulls, engines, suspension, and tracks.