M12 GMC is a United States Artillery unit.
The start of war in Europe saw the American Artillery thinking about creating Gun Motor Carriage. This vehicle built on a M3 chassis to carry the 155 mm M1918M1, the French 155 mm GPF cannon built in America. Production started in August 1942 with first vehicles serving as trials vehicle and training vehicles. The rest were placed in storage. These were placed in active service early enough for the Normandy Landings. The M3 chassis were modified to have a much M4 components as possible since M3 were out of service.
M12 were used six artillery battalions (557th, 558th, 981st, 987th, 989th, and 991st) with three four-gun batteries. These were used in artillery barrages and counterbattery fire. With their tracks M12 Battalions were able to keep up with Armored forces during the pursuit of the retreating Germans.
991st Field Artillery Battalion supported the 3rd Armored during the breakout since Self-propelled Artillery pieces were able to keep up with the Armored Division.
989th Field Artillery Battalion served in XV Corps which the 4th Armored was in during the Normandy Campaign.