The M3 was the first US medium tank to see service during World War II, and precursor to the M4. It was initially developed as a stopgap design while the Sherman was in development, and was characterized by an unconventional tank design. The main gun, a 75-mm M2, was fitted on a side sponson which gave it limited traverse but it was still the most powerful gun fitted on a Western tank at the time. In contrast, the turret was armed with a much weaker 37-mm gun, and the overall high profile of the vehicle (a problem also shared with the M4) was a noticeable hindrance on the battlefield. Nevertheless, the M3 was relatively well protected for its day, and its 75-mm gun caused quite a shock to German troops when it was first encountered in North Africa (against the British 8th Army), as it could defeat any panzer then in service. In British service, the M3 was known as the Lee and the Grant, the latter which had various modifications stemming from a revised turret. M3s were gradually phased out of front-line service once the M4 was introduced, but it was still used in the Pacific for bunker-busting duties and for infantry support. The M3 was also supplied as Lend-Lease to the USSR as well as other British Commonwealth forces.

Experience in the early years of the war showed that the 37-mm gun in the M2 medium tank would be insufficient to defeat German tanks, this resulted in the Medium Tank M3, a stopgap measure prompted by the urgency to supply the British Army with tanks (due to the refusal of producing British tanks in the US). The basic M3 was the most widely produced version and it was followed by the M3A1 with a rounded cast upper hull rather than the bolted hull of the original. This was then followed by the short-lived M3A2 with a welded hull, the M3A3 with twin GM 6-71 diesel engines. The M3A4 was fitted with a Chrysler A57 multi-bank engine but this required hull and suspension modifications. Notably, it had the side doors removed and reverted to the bolted hull of the original M3. The last tank variant was the M3A5 which went back to the GM diesels and the riveted hull of the M3. British modifications to the basic M3 resulted in the Grant I. This included a new cast turret without the machine gun cupola (to reduce height), and with radio equipment that allowed for one less crew member. British tanks using US turrets were known as the Lee I (M3), Lee II (M3A1), Lee IV (M3A3), Lee V (M3A3 diesel), and Lee VI (M3A4), as well as the Grant II (M3A5) which maintained the US turret. Other variants included the M7 Priest Howitzer Motor Carriage, M12 Gun Motor Carriage, and the M31 Tank Recovery Vehicle. Other British version included the Grant ARV Armored Recovery Vehicle, Grant Scorpion mine flail vehicles, and the Grant CDL with the turret gun replaced by a carbon-arc searchlight.