The M6 was a heavy tank that was developed by the United States during WWII. It was at first designed to be a heavier counterpart to the M3 Medium Tank, with a similar armament, with a hull mounted 75mm gun and a turret mounter 37mm antitank gun. But, in 1940, that specification was recalled, and a new one was issued: a more conventional layout of a turret mounting a 76mm antitank gun with a coaxial 37mm antitank gun, with maximum armor protection of 5.23 inches and a speed of 22mph. In February 1941, four different prototypes were built, with different combinations of transmission types and hull constructions, designated T1E1 through T1E4. After testing, a cast hull version with a torque converter was standardized as the M6 Heavy, a welded hull version of that as the M6A1, and a cast hull version with electric transmission was semi-officially called the M6A2. The A2 model was the most numerous, but only a total of 40 M6 type tanks were ever built. The project was cancelled when it was decided to concentrate on fewer heavy tank designs, sealing the fate of U.S. heavy tanks until the M26 finally got through the red tape in late 1944.