Known as the T5 medium tank in testing at Aberdeen Proving Ground in 1939, the M-2 Medium Tank was first produced at Rock Island Arsenel in 1939 and entered service in the spring of 1940. Its 37mm cannon and eight machine guns were protected by only 32mm of armor (maximum) and the tank was underpowered for its weight. At the same time, German forces introduced the heavily armored Panzer-Kampfwagen IV, with a 75mm gun. The M-2 Medium Tank was quickly upgraded to the M2A1 with more armor and a more powerful engine as well as a redesigned turret, but a new tank was needed.
The M2 Medium Tank was lagging tank development in Europe and was obsolete before it could be used in World War II other than for training. Only 18 M2 and 94 M2A1 medium tanks were produced before being superseded by the M3 Lee. However, some aspects of the M2 medium tank design were found to be effective, particularly the sloped frontal armor (hull glacis plate) which found its way into M3 Lee and M4 Sherman medium tanks.