Observing events in Europe, American tank designers realized that the Light Tank M2 was becoming obsolete and set about improving it. The upgraded design, with thicker armor, modified suspension and new gun recoil system was called "Light Tank M3". Production of the vehicle started in March 1941 and continued until October 1943. Like its direct predecessor, the M2A4, the M3 was initially armed with a 37 mm M5 gun and five .30-06 Browning M1919A4 machine guns: coaxial with the gun, on top of the turret in an M20 anti-aircraft mount, in a ball mount in right bow, and in the right and left hull sponsons. Later, the gun was replaced with the slightly longer M6, and the sponson machine guns were removed. For a light tank, the Stuart was fairly heavily armored. It had 38 mm of armor on the upper front hull, 44 mm on the lower front hull, 51 mm on the gun mantlet, 38 mm on the turret sides, 25 mm on the hull sides, and 25 mm on the hull rear.
A M3A1 going through water obstacle, Ft. Knox, Ky.
The M3 and M3A1 variants were powered by an air-cooled radial engine, either a gasoline-fueled 7-cylinder Continental W-670 (8,936 built) or a 9-cylinder Guiberson T-1020 diesel (1,496 built). Both of these powerplants were originally developed as aircraft engines. Internally, the radial engine was at the rear and the transmission at the front of the tank's hull. The propeller shaft connecting the engine and transmission ran through the middle of the fighting compartment. The radial engine's crankshaft was positioned high off the hull bottom and contributed to the tank's relatively tall profile. When a revolving turret floor was introduced in the M3 hybrid and M3A1, the crew had less room. A further 3,427 M3A3 variants were built with modified hull (similar to the M5), new turret and the Continental W-670 gasoline engine. In contrast to the M2A4, all M3/M5 series tanks had a trailing rear idler wheel for increased ground contact.
Crews from Company D, 761st Tank Battalion
, stand by awaiting call to clean out scattered Nazi machine gun nests in Coburg
To relieve wartime demand for the radial aero-engines used in the M3, a new version was developed using twin Cadillac V8 automobile engines and twin Hydra-Matic transmissions operating through a transfer case. This version of the tank was quieter, cooler and roomier; the automatic transmission also simplified crew training. The new model (initially called M4 but redesignated M5 to avoid confusion with the M4 Sherman) featured a redesigned hull with a raised rear deck over the engine compartment, sloped glacis plate and driver's hatches moved to the top. Although the main criticism from units using the Stuarts was that it lacked firepower, the improved M5 series kept the same 37 mm gun. The M5 gradually replaced the M3 in production from 1942 and, after the M7 project proved unsatisfactory, was succeeded by the Light Tank M24 in 1944. Total M5 and M5A1 tank production was 8,885; an additional 1,778 M8 75 mm howitzer motor carriages based on the M5 chassis with an open-top turret were produced.