The M3 75mm Gun Motor Carriage was an interim design for a tank destroyer with a 75mm gun mounted on the back of a M3 half-track personnel carrier. In 1940-41 the US Army Ordnance worked on a number of designs for tank destroyers armed with the standard 37mm anti-tank gun, but by 1941 this gun was obsolete and although over 5,000 M6 37mm gun motor carriages were produced, work had already moved on to vehicles with heavier guns.
The first design was the T1 3in gun motor carriage. This mounted a 3in gun on the chassis of a Cleveland Tractor Co high-speed tractor. Work got underway towards the end of 1940, but although the design was standardized in November 1941 as the M5 3in Gun Motor Carriage it was never a popular design.
In June 1941 work began on the T12 75mm gun motor carriage. This was another interim design, this time matching a 75mm gun with a M3 half-track personnel carrier. The personnel carrier turned out to be a good base for conversion, and the design was standardised as the M3 75mm Gun Motor Carriage in October 1941, the month before the M5 was standardised.
The M3 was a simple conversion. The gun and crew were mounted in the open back of the M3 half-track, with the gun firing forwards over the bonnet. The gun chosen was the M1897A 75mm, an American version of the famous French '75' of the First World War. The gun was mounted on a box that was welded onto the floor of the half-track. Nineteen rounds could be carried in a three-row rack built into the box and another forty were stored in bins built into the floor.
The M3 was quickly put into production. A series of 86 pilot-vehicles was built in August-September 1941 with the T12 designation. An improved gun shield was then developed and production of the standard model began in February 1942. A total of 2,202 were built by the time production ended in April 1943. Early vehicles used the M2A3 gun mount, but the supply of this mount ran out and later vehicles used the similar M2A2 gun mount. These were given the designation M3A1 75mm Gun Motor Carriage. The two models are hard to identify visually.
The T12 pilot vehicles were the first American tank destroyers to see combat. In November 1941 the US Army began to form a Provisional Tank Group in the Philippines. This was equipped with the M3 light tank and with 50 of the 86 pilot models of the T12. These vehicles were sent to the Philippines in November-December 1941 and formed into three battalions in the Provisional Field Artillery Brigade. There were very few encounters between the new tank destroyers and Japanese tanks and instead they were used as mobile artillery, providing direct fire to support the rearguard during the retreat to Bataan and during the fighting in the Bataan peninsula. As with the light tanks, some of the T12s were captured and later used against the Americans when they returned to the Philippines.
In 1942 the M3 75mm GMC and M6 37mm GMC were the only tank destroyers available to the US Army. It was decided to deploy them in mixed battalions that would be equipped with both types and in heavy self-propelled battalions that would only use the M3.
The first of the official tank destroyer battalions to be deployed was the 601st Tank Destroyer Battalion. This was a mixed unit, equipped with both the M3 and the M6. It took part in Operation Torch, the invasion of North Africa. The M6 was judged to have been a failure, but the M3 was more successful. In March 1943 the 601st helped repel an attack by about 100 German tanks at El Guettar. They claimed to have destroyed thirty German tanks, including two Tiger Is, at the cost of twenty-one M3 75mm GMCs.
The tank destroyer was a controversial weapon. General McNair, commander of Army Ground Forces, believed that they were a powerful, low cost weapon that just needed to be used carefully and not wasted in the infantry support or assault gun roles. He believed that tanks shouldn't be used to fight other tanks, and remained a supporter of towed anti-tank artillery even after it became very clear that it was vulnerable to enemy fire and slow to deploy.
In contrast General Devers, commander of the Armored Force during Operation Torch, considered the separate tank destroyer force to not be a practical concept, while Patton and Bradley both wanted them replaced with proper fully armoured tanks, which were more flexible on the battlefield. Their argument was that it wasn't always possible to keep the tank destroyers in reserve until the exact right moment to use them, and that sometimes every weapon available would have to be used.
Although the M3 had performed acceptably in North Africa, the tank destroyer commanders all agreed that the new M10 3in GMC, which carried its gun in an open topped turret on the chassis of an M4 Sherman medium tank, was greatly superior to the M3. As a result the M3 was withdrawn from US army Tank Destroyer units as quickly as possible. It was declared Limited Standard in February 1944, indicating that it should only be used in unusual circumstances, and declared obsolete in August 1944.
The M3 remained in service with the US Marine Corps into 1945. They used it in the divisional special weapons companies, and they were used in the direct fire support role. They were used at Cape Gloucester in December 1943, where they were used by the 1st Marine Division, on Bougainville and Saipan in 1944, and were still in use during the fighting on Iwo Jima and Okinawa in 1945. The M3 was used to equip the special weapons companies of the Marine divisions, and were called the SPM (Self-propelled mount) in Marine service.
The M3 was also used by the British. It was used in the heavy troops of armoured car units to provide fire support, ironically one of the roles that General McNair had argued against. The British M3s were used in Italy from 1943 and in smaller numbers in France in 1944. They were often seen lined up and acting as artillery. In British service the M3 was given the designation 75mm SP, Autocar.