Developed in 1935 at Rock Island Arsenal, the M-2 Light Tank was intended for infantry support, based originally on the Vickers 6-ton British light tank. Called T2E1 before standardization, its only weapon in the first model was a single .50-cal. machine gun in a small turret. Almost immediately, the armament was upgraded by providing a second turret with a .30 cal. machine gun in a twin turret configuration (quickly called "Mae West" by the troops.) By the end of its life cycle, events in Europe showed that the M-2 light tank required more than machine guns. The M2A4 replaced the turret machine guns with the M5 37mm gun in a single turret, along with other improvements.
The shape of the turret changed with each model variant, plus additional changes within the production cycle. Here is a summary:
Cylindrical cross-section single turret
Twin MG turrets, Mae West, initially cylindrical, later small turret had 7 flat sides, large turret 8
Same as A2 but twin turrets wider spaced
Single turret with eight sides
Raised M2A3 Rear Idler Wheel
In March 1941, the M-3 Stuart light tank replaced the M-2 light tank. It can be difficult to tell the M-2A4 and M-3 light tanks apart since the early M-3 Stuart was quite similar to the M-2s. One distinguishing feature is the rear idler wheel, raised on the M2 Light Tank but rides on the ground on the M3 Stuart. The M2A4 also uniquely has seven pistol ports on the turret sides.
When the United States entered World War II in December 1941, the M2A1 through M2A3 light tanks were relegated to training. The US Marine Corps landed 50 M2A4 Light Tanks on Guadalcanal with the USMC 1st Tank Battalion. These remained in service in the Pacific Theater until 1943. That is the only known combat service by the M-2 Light Tank with US forces although the British received 36 M2A4 light tanks, possibly used in North Africa or CBI.