The project to develop a tank gun of 105mm (4.1  in) in caliber was first started during WWII in order to compete with increasing heavily armored German tanks. After the invasion of Europe, the Ordnance Department believed there would be a need for heavily armored tanks equipped with powerful armament to break through fortified areas. They proposed that existing M6 heavy tank prototypes be up-armored and mount the new 105mm T5E1. These modified T1E1s with the T5E1 gun in enlarged turrets (taken from the proposed T29 project) were designated M6A2E1. The M6A2E1 project did not proceed when Eisenhower rejected its use in Europe on August 18, 1944, but two M6A2E1s – without the increased armor – were used to test the armament and gun for the upcoming T29 Heavy Tank.