With events in Europe in 1940, the Army realized the M2 was inadequate and the decision was made to modernize it. A new design with thicker armor was standardized on July 5, 1940. Production started in March 1941 at the American Car & Foundry. It was based on the M2A4, but had thicker armor that was homogeneous rolled and the idler wheel was placed on the ground to act as another road wheel to help distribute the weight.

Initially the engine was the Continental but shortages caused it to be replaced by the Guiberson T-1020 diesel, and fitted into 500 M3s.

Tracks could often last 1,000 miles, whereas other models required replacement at 500 miles.

During production the riveted turret was replaced by a welded one 7-sided one. Also in early 1942 an all welded hull was produced.

It had volute spring suspension with the rear idler on the ground. This reduced the ground pressure and gave support to the rear of the tank.

When production ended in October 1943, 13,859 were built.

The engines were in the rear with the drive going to the front sprockets, which was controlled by differential steering.

The driver was on the left and the hull gunner/assistant driver on the right.7

Some vehicles also had two 25 gallon jettisonable fuel tanks mounted on the side of the rear deck based on British experience in North Africa.

During the retreat in Burma in 1942, Stuarts of the 2nd Royal Tanks went 2,400 miles in eleven weeks with very little maintenance.

M3s were used by the 8th King Royal Irish Hussars in August 1941 in Egypt. Fought their first battle at Sidi Rezegh on November 19, 1941.

Organization

There were 72 M3s in a Marine Light Tank Battalion.