The M12 GMC, nicknamed 'King Kong', was a self-propelled gun based around the French-designed 155-mm M1918 gun used by both the French and US Armies since World War I. Like the smaller M7 Priest, the M12 was based on the M3 Lee chassis with some structural modifications which included moving the engine from the rear to the center of the vehicle in order to make room for the gun mount. Recoil was partly ameliorated with the use of a recoil spade in the rear which improved stability. The gun crew operated in an open top superstructure which had minimal protection and no defensive armament was provided. Among its drawbacks was limited ammunition of just 10 rounds, which necessitated the development of a complementary M30 ammunition carrier. The M12 was built in small number and equipped just six artillery batallions after D-Day. They were occasionally armed with armor-piercing rounds to attack enemy fortifications particularly during the attack against the Siegfried Line. It was eventually replaced by the M40.

The prototype T6 was built in early 1942 and was initially rejected, but later entered production in limited numbers as the 155 mm Gun Motor Carriage, M12. No variants were built. The M30 Cargo Carrier (initially T14) was designed to carry 40 rounds of ammunities and provide some defensive fire in the form of an M2 Browning machine gun.