From the USA the Soviet Union received 1386 M3 Medium tanks of various models. They were not widely liked, being inferior to the T-34. A number were captured by the Germans, who then used them against Soviet forces.
Events in Europe after mid-1940 when 1000 M2A1 Mediums were ordered demonstrated that a 37mm (1.46in) gun was an inadequate main armament for a battle tank. German tanks with 75mm (2.95in) cannon were sweeping all European tanks - mostly armed with 37mm (1.46in) or 6pdr (57mm (2.24in)) guns - before them. The US Ordnance Department wanted to fit a 75mm (2.95in) to the M2A1 Medium, but there was no turret available that was able to take this gun and fit into the limited space atop the barbette.
As a stop-gap measure based upon experience with the earlier T5E2, an installation was devised whereby an M2 75mm (2.95in) gun was mounted in the right side of the hull of a modified M2A1. The M2 gun was developed from the standard French-designed US Army howitzer and could penetrate 60mm (2.36in) armour sloped at 30 degrees at a range of 500m (547yd), making it a better weapon than contemporary German tank guns. The main gun had only limited traverse, 30 degrees in azimuth and 29 degrees in elevation. The 37mm (1.45in) was rotatable by hand, a 360-degree sweep taking 20 seconds.
A wooden mock-up of the new interim tank was completed in August 1940. Configuration was basically that of the M2A1, with the 7Smm (2.9Sin) in the place of the right machine-gun sponsons and with a new 37mm (1.45in) turret on top. The mock-up board ordered several changes, including removal of the remaining machine-gun sponsons, and the lowering of the turret.
The M3A1 was built by the American Locomotive Company with a cast upper hull, rather than a riveted construction with large rivets that would fly about inside when the tank was struck. In total, 300 were built. Only 12 M3A2s with a welded hull and petrol engine were built before production switched to the similar M3A3 with a twinned GMC 6046 diesel truck engine. This offered better economy, range and combat safety, but required many changes at the rear, including armoured radiators, and raised the weight by 1179kg (1.16 tons). The M3A5 was a version (332 built) with riveted hull and diesel powerplant.
The M3 was built by several US companies, including Alco, Baldwin, Detroit, Pressed Steel Car and Pullman-Standard, and it became the first US medium tank to go into volume production. Factories in Canada also built 1100 M3s. At first, the M3 had a seven-man crew, with a loader and gunner for each main weapon and a radio operator. This latter position was soon deleted and the radio given to the driver. The suspension inherited from the M2 turned out to be inadequate for the heavier M3, and was re-designed with heavier springs. Ammunition stowage was 46 rounds of 75mm (2.95in), 178 of37mm (1.46in), and 9200 rounds of machine-gun ammunition. The USSR received M3A3 and M3A5 tanks but, with high silhouettes and archaic configuration, were unpopular, and nicknamed the 'Grave for Seven Brothers'.
Although the US Army Armored Force would have preferred to develop a new light tank with a gun of up to 75mm (2.95in) calibre, its pressing need was to re-arm before the United States became embroiled in the European war. As a result, the next development was an evolutionary step, rather than a revolutionary leap in tank design.
The US Army chose to modernize the M2A4. The major change was to increase the armour thickness on the upper surfaces and replace the brittle, face-hardened steel with homogenous rolled plate. The extra weight required a beefed-up suspension, specifically a new idler wheel mounted at ground level. Standardized in July 1940, production of the M3 Light Tank began in March 1941, directly after the last M2 was rolled out. During production, the M2's turret with its large rivets was replaced by a welded unit with reduced weight and afforded better ballistic characteristics. Jettisonable external fuel tanks were added to increase range, and a simple gyro-stabilizer was fitted to the 37mm (1.46in) gun. This last addition was a major advance. Based on a system for naval guns, the gyroscopes held the gun in elevation even as the tank moved across undulating terrain. This allowed the tank to fire without stopping first, and was a major tactical advantage, as no Axis (or other Allied) nation developed such a system during World War II.
The M3A1 version eliminated the remotely operated sponson machine guns and introduced an all-welded hull during production. Other improvements were made to the sights, vision equipment, radio and crew intercom. A gyrostabilizer was fitted to the 37mm (1.46in) gun, the first such equipment to enter service. The turret cupola was removed to lower the vehicle's silhouette. The M3A3 had a redesigned all-welded hull with longer side sponsons, and it entered production in 1943. The Soviet Union received 1676 Guiberson diesel-powered M3A1 Light Tanks under Lend-Lease. A small number were shipped from British stocks, but the bulk carne from the USA. Soviet crews criticized the high silhouette of the tank and also ridiculed the hull machine guns.
The M3 Medium was always regarded as an interim solution to US tank needs while a turret could be produced to mount a 75mm (2.95in) gun. Improvements in casting technology and success with the 37mm (1.46in) turret on the M3 encouraged development of a compact, curved unit, which was drawn up by March 1941. The Armored Force Board was offered five options based on this turret and, in April 1941, selected the simplest, which involved fitting it to a modified M3 hull and chassis. The T-6 Medium mock-up was approved in May, and the pilot model delivered to Aberdeen in September 1941. One change was the elimination of a machine-gun cupola. The T-6 hull was of welded construction, as in later M3s, and its prominent side hatch was deleted in the production model.