After observing the new and often decisive use of armored vehicles on both sides during the French campaign of 1940, the US Army decided that it required a 75 mm self-propelled gun, based on the chassis of the M3 half-track.

The T12/M3 first served in the Philippines Campaign in 1942 with the Provisional Field Artillery Brigade in the anti-tank and the fire-support role.

[1][2] It had a White 160AX 147 horsepower (110 kW), 386 cubic inch (6,330 cc),[3] 6-cylinder engine with a compression ratio of 6.44:1.

It had a 150-mile (240 km) range,[4] 60 US gal (230 L) fuel tank, a speed of 47 mph (75 km/h), and a power to weight ratio of 14.7 hp per ton.

[5] After the fall of France, the U.S. Army studied the reasons behind the effectiveness of the German campaign against the French and British forces.

The Army had a number of M1897A5 guns, sufficient for the mass-production for such a weapon, and the M3 half-track was coming into production.

After some debate, the Army decided to place M1897A5 guns on the M3 half-track chassis,[6] which was designated the T12 GMC.

During the early part of the campaign, the vehicle was used to provide direct covering fire and anti-tank support.

The M3 GMCs, which were designed for ambushing tanks, proved to be inadequate for this task in the battles of Sidi Bou Zid and Kasserine Pass, mainly due to poor tactics.

Some M3s also saw service in Allied invasion of Sicily (Operation Husky), but by that time, the M10 tank destroyer had replaced it in the U.S.

A small batch of 170 vehicles was supplied to Britain, which used them in their Armoured Car Regiments which were the reconnaissance units of the armored formations.