The Army Ground Forces initially rejected the design as unnecessary, but after the Artillery Board supported the Ordnance Department in asking for 50 units, 100 were authorized and built by March 1943.

[2] Limited storage space meant that only 10 155 mm projectiles and propellant charges could be carried on the vehicle.

[1] They were assigned to six armored field artillery battalions (the 258th, 557th, 558th, 987th, 989th, and 991st) and employed successfully throughout the campaign in North-West Europe.

Although designed primarily for indirect fire, during assaults on heavy fortifications, the M12s were sometimes employed in a direct-fire role, such as in the Allied assault on the Siegfried Line, where the M12 earned its nickname "Doorknocker" thanks to the 155mm cannon's ability to fire armour piercing rounds through seven feet of concrete at ranges up to 2,000 yards (1,830 meters).

[4][dubious – discuss] The vehicle was also dubbed "King Kong" by American operators due to the raw power of its gun.