The 90 mm gun M1/M2/M3 was an American heavy anti-aircraft and anti-tank gun, playing a role similar to the German 8.8cm Flak 18. It had a 3.5 in (90 mm) diameter bore, and a 50 caliber barrel, giving it a length of 15 ft (4.6 m). It was capable of firing a 3.5 in × 23.6 in (90 mm × 600 mm) shell 62,474 ft (19,042 m) horizontally, or a maximum altitude of 43,500 ft (13,300 m).

The 90 mm gun was the US Army's primary heavy anti-aircraft gun from just prior to the opening of World War II into 1946, complemented by small numbers of the much larger 120 mm M1 gun. Both were widely deployed in the United States postwar as the Cold War presented a perceived threat from Soviet bombers.