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 diameter bore, and a 50 caliber barrel, giving it a length of . It was capable of firing a shell horizontally, or a maximum altitude of .
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. The anti-aircraft guns were phased out in the middle 1950s as their role was taken over by surface-to-air missiles such as the MIM-3 Nike Ajax.
As a tank gun it was the main weapon of the M36 tank destroyer and M26 Pershing tank, as well as a number of post-war tanks like the M56 Scorpion. It was also briefly deployed from 1943–1946 as a coast defense weapon with the United States Army Coast Artillery Corps. Each gun cost roughly $50,000 to make in 1940 and utilized up to 30 separate contractors to manufacture.
History
Prior to World War II, the primary US anti-aircraft guns were the 3-inch M1918 gun (76.2 mm L/40) and 3-inch anti-aircraft gun M3 (76.2 mm L/50), a widely used caliber for this class of weapon. Similar weapons were in British, Soviet and other arsenals. There had been several upgrades to the weapon over its history, including the experimental T8 and T9 versions developed in the early 1930s, that were intended to enter service later in that decade.
However, the US Army became interested in a much more capable weapon instead, and on June 9, 1938, it issued a development contract calling for two new guns, one of 90 mm, which it felt was the largest possible size that was still capable of being manually loaded at high elevations, and another, using assisted loading, of . On 18 August 1938 the development of the Mounting T1 was approved as well. The new design seemed so much better than developments of the older three-inch that work on the three-inch T9 was canceled in 1938, just as it became production-ready. On 21 March 1940, the second development of the 90 mm design, the T2, was standardized as the 90 mm M1, while its larger cousin became the 120 mm M1 gun.
A few hundred M1s were completed when several improvements were added to produce the 90 mm M1A1, which entered production in late 1940, and was accepted as the standard on May 22, 1941. Since national arsenals had limited capacity, production of first 300 gun mounts was contracted to Allis-Chalmers in November 1940, with the first unit shipped in September 1941. The M1A1 included an improved mount and spring-rammer on the breech, with the result that firing rates went up to 20 rounds per minute. Several thousand were available when the US entered the war, and the M1A1 was their standard anti-aircraft gun for the rest of the conflict. Production rates continued to improve, topping out in a few thousand per month.
Like the German 88 and the British QF 3.7 inch AA gun, the M1A1 was used against tanks in combat but, unlike the others, it could not be depressed to fire against them. On September 11, 1942, the Army issued specifications for a new mount to allow it to be used in this role, which resulted in the 90 mm M2, introducing yet another new mount, one that could be depressed to 10 degrees below the horizontal and featured a new electrically-assisted rammer. It became the standard weapon from May 13, 1943.
Anti-aircraft operation
In anti-aircraft use, the guns were normally operated in groups of four, controlled by the M7 or M9 gun director or Kerrison predictors. Radar direction was common, starting with the SCR-268 in 1941, which was not accurate enough to directly lay the guns, but provided accurate ranging throughout the engagement. For night-time use, a searchlight was slaved to the radar with a beam width set so that the target would be somewhere in the beam when it was turned on, at which point the engagement continued as in the day. In 1944, the system was upgraded with the addition of the SCR-584 microwave radar, which was accurate to about 0.06 degrees (1 mil) and also provided automatic tracking. With the SCR-584, direction and range information was sent directly to the Bell Labs M3 gun data computer, and M9 director, which could direct and lay the guns automatically, all the crews had to do was load the guns.