The 155mm Gun Motor Carriage M12 was a US self-propelled gun developed during the Second World War. Only 100 were built; 60 in 1942 and a further 40 in 1943. It mounted a 155mm gun M1917, M1917A1 or M1918 M1, depending upon availability, a weapon derived from the nearly identical French 155mm GPF gun of WW1 vintage. The M12 was built on the chassis of the M3 Lee tank (some sources claim that later M12 used the M4 Sherman chassis but this might be a confusion with the M12's use of "Sherman-style" bogie trucks with trailing idlers). It had an armoured driver's compartment, but the gun crew were located in an open topped area at the back of the vehicle. An earth spade (similar to a bulldozer blade) at the rear was employed to absorb recoil. This layout - large gun mounted in an open mount at the rear, with a spade - was the pattern adopted for many years by other heavy self-propelled artillery.
During 1943 the vehicles were used for training or put into storage. Before the invasion of France 74 M12 were upgraded in preparation for combat operations. They were employed successfully throughout the campaign in NW-Europe. Although designed primarily for indirect fire, during assaults on heavy fortifications the M12s were sometimes employed in a direct-fire role.
Limited storage space meant that only 10 projectiles and propellant charges could be carried on the vehicle. Given this, a similar vehicle, but without the gun, was produced as the Cargo Carrier M30. This was designed to transport the gun crew and additional ammunition. In operational conditions the M12 and M30 would serve in pairs. The M30 was armed with a .50cal Browning M2 machine gun. It could carry 40 rounds of 155 mm ammunition.