The M6 was an outgrowth of a 1940 U.S. Army Ordnance Corps program to develop a 50-ton heavy tank. After the initial, multi-turreted design proved too excessive, a single three-man turret bearing 76.2mm and 37mm guns was adopted. The tank also carried four Browning machineguns, three fixed and one flexible. In addition to the commander, gunner and loader in the turret, the tank had a driver, assistant driver, and an ammunition passer. The tank was equipped with a Wright radial gasoline engine and a torque converter or electric transmission and designated M6 (cast hull) and M6A1 (welded hull). While it might have fared well in combat, the heavy tank's exorbitant cost, high silhouette, reliability problems and difficulty in transport made it vulnerable to enemies in the Army staff. In the end, only forty M6 type tanks were produced, and the design was declared obsolete by late 1944. Created during the testing phase of the M6, this TM 9-721 technical manual reveals a great deal about the weapon's design and capabilities. Intended as a manual for those charged with operation and maintenance, this manual shows the basics of how to drive the tank and take care of its engine, cooling, power train and other systems. Originally printed in 1943 and labeled restricted, this manual was declassified long ago and is here reprinted in book form. Care has been taken to preserve the integrity of the text.