The Chief of Ordnance ordered a pilot model to have a 155 mm M1918M1 gun put onto a M3 medium chassis. This was designated the T6 and work began on it in June 1941.
The engine was located in the front of the M12.
The vehicle commander and driver sat in a compartment at the front of the M12.
The driver and assistant driver had direct and indirect vision. In the M12 there were seats for 4 other crewmen. There was also a hinged platform for the crew for firing.
The M12 provided armor protection for the crew.
The 155 mm M1918 was formerly a French gun that was used as towed artillery in World War I. After World War I they were placed into storage.
The 155 mm gun fired a 95 lb shell at a muzzle velocity of 2,380'/sec and a range of 18,000 yards.
The M12 had a spade at the rear to help stabilize against the recoil of the gun. This could be raised when moving.