In order to develop a mobile anti-aircraft weapon, several 0.5 inch (12.7 mm) twin machine gun mounts were tested on the chassis of the M2 half-track including Bendix, Martin Aircraft Company, and Maxson. The Maxson M33 turret mount was preferred and - on the larger M3 half-track (T1E2) - was accepted for service in 1942 as the M13 Multiple Gun Motor Carriage. The mount was also used on the similar M5 half track as the M14 Multiple Gun Motor Carriage.
Experimentally, the Quadmount was also tested in 1942 on a M3 Light Tank in place of the tank's turret but the project was not proceeded with.
Even as production of the two MGMC vehicles was underway, there was work to increase the firepower. Re-working the M33 to take four machine guns gave the M45 mounting.
The M45 Quadmount was the principal weapon (along with the 40mm Bofors gun) of highly mobile anti-aircraft artillery battalions deployed in the European Theater during World War II. These battalions provided invaluable air defense to much larger units, particularly field artillery. The M45 Quadmount units served as a very strong deterrent to strafing runs by enemy warplanes as, in addition to their gross firepower, the four .50 caliber guns were capable of being "tuned" to converge upon a single point at distances which could be reset while in use. Multiple-gun mounts were developed for the M2 Browning because the M2's rate of fire (450-550 rounds per minute) was too low for anti-aircraft use.
The M45 found use throughout the war as a land-based weapon, particularly during the Battle of the Bulge. Although the Allies achieved air supremacy by the invasion of Normandy in June 1944, German attack runs were still a threat. German fighter-bombers could approach and attack at low altitude and then quickly retreat to avoid Allied fighters. The Luftwaffe also mustered a large number of planes for the massive Operation Bodenplatte New Year's Day 1945 attack. On March 7, 1945 when the U.S. Army discovered the Ludendorff Bridge intact across the Rhine, M45-armed U.S. half-tracks defended the American troops crossing the bridge on March 9, 1945 to great effect. At Oppenheim, when the Allies were gathering to make a massive push after crossing the River Rhine (March 1945), 248 German warplanes were used in an effort to destroy the bridge first. U.S. Army anti-aircraft artillery battalions massed, shot down 30% of the attacking force mainly with M45 Quadmounts and prevented the bridge from being touched before the U.S. Third Army went into Germany.
The M45 Quadmount was of little use against the new, fast-flying planes of the Jet Age. However, it was used against infantry targets in US post-war service.
The TCM-20 was a postwar Israeli development of the M45 mount, equipped with two 20mm Hispano-Suiza HS.404 cannons in lieu of machine guns. In frontline Israeli service, it was replaced by the M163 Vulcan Air Defense System in the 1970s, but some reserve units still used TCM-20s into the 1980s. The weapon was also exported to several third-world countries.