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. One aspect that was highlighted by this study was the use of self propelled artillery; however, by 1941, there was little available in the U.S. Army's arsenal that could be used in such a role. 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, which was designated the T12 GMC. The M1897A5 gun was originally adapted for the M3 chassis by placing it in a welded box riveted to the chassis behind the driver's compartment. It was accepted by the Army on 31 October 1941.
The T12/M3 GMC first saw action with the U.S. Army in the Philippines in 1941–42, six months after it was designed. Three battalions of the Provisional Field Artillery Brigade operated T12s against the Japanese when they invaded the Philippines. During the early part of the campaign, the vehicle was used to provide direct covering fire and anti-tank support. The Japanese captured a few vehicles in 1942 and used them in the defense of the Philippines.
By 1942, M3 GMCs were being used by tank destroyer battalions in the North African Campaign, each of which consisted of 36 M3s and four 37 mm M6 GMCs. 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. Nevertheless, the M3 was later used in the Battle of El Guettar with success, claiming 30 German tanks, including possibly two Tiger tanks, at the cost of 21 M3s. 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. Army. A total of 1,360 M3 GMCs were also converted back into M3A1 half-tracks.
The M3 also served with the U.S. Marines in the Pacific Theater of Operations and was first used in the invasion of Saipan. It proved highly effective against the Type 95 Ha-Go and the Type 97 Chi-Ha, in the fight against the Japanese 9th Tank Regiment on Saipan. It also served in the Battle of Peleliu and the Battle of Okinawa, and many other conflicts in the Pacific.