After the invasion of the Low Countries and France in 1940 the Chief of Infantry requested two types of heavy tanks to be developed. One would be similar to the M2 Medium with 3" of armor, a hull mounted 75 mm gun, and a turret with a 37 mm or 50 mm gun. This eventually led to the M3 Medium. The second, which became the M6 Heavy, was not to be over 80 tons, have 3" of armor, a hull gun from 75 mm to 105 mm, and a 37 mm or 50 mm in the turret. Spread throughout the vehicle were to be eight machine guns.
In 1940 this was changed to have the main gun be a 76 mm and have the 37 mm coaxially mounted.
The final drive was mounted behind the engine. The transmission was connected by a flexible coupling without the use of a propeller shaft. There was a pedal that occupied the position normally occupied by the clutch petal, and this served as a transmission brake pedal.
The 3" gun was mounted with the 37 mm gun. A gyrostabilizer was provided as well.
There were 4 bogie assemblies on each side. Each had 4 bogie wheels. Two riding on the outside half of the track and 2 on the inside. Two springs were mounted on each assembly. The driving sprocket was at the rear. There was a main idler in the front and there was a nonadjustable idler between the main idler and the front bogie assembly.
Six periscopes were provided. There were 4 escape doors.
After over 3,500 miles of testing, in April 1942, it was decided to completely redesign the brakes and cooling system due to flaws that were found. The new brakes were similar to train engine brakes as they were hydraulically operated disc brakes applied to the differential.
The Armored Board decided that the M6 was too heavy and unreliable so they recommended that the contracts should be cancelled. The Ordnance Department said it could modify the design of the M6 to rectify these problems but the Army Ground Forces decided it didn't want the M6. In September 1942 orders were reduced to 115, with 50 of those going to the British Army. Also, the Armored Forces felt that the M6 would take up too much shipping space as two M4 Mediums could fit in the same space a single M6 would fit.