The M15 CGMC (Combination Gun Motor Carriage) was the first major anti-aircraft vehicle to be produced in large numbers for the US Army in World War II. It was based on the chassis of the M3 half-track, with the crew compartment removed and in its place a large, fully traversible M42 mount was installed. The mount was open top and armored on all sides although its cramped nature meant that the frontal armor was eventually removed to improve visibility. Armament consisted of a 37-mm M1 cannon along with twin 0.50-in Browning machine guns; these were initially located above the cannon but later relocated below it when the newer M54 mount was introduced. The M1 cannon had a rate of fire of 120 rounds per minute and all three guns could be fired independently. Typically, tracer rounds from the Browning guns would be used to target an enemy aircraft before bringing the main gun into action; in many cases, aircraft would be fooled into thinking they were out of range of the machine guns only to be hit by the longer ranged cannon. The M15 saw action for the first time during the invasion of North Africa and saw continued service throughtout the war alongside the machine gun-only M16, not only in the anti-aircraft role but also providing fire support to ground forces. Only a small number were provided as Lend-Lease to the USSR, the rest remained in US service as late as the Korean War.

The initial prototype T28 began development in the early1940s and was based on the slightly smaller M2 half-track. This was changed to the M3 in the T28E1 which entered production as the M15 with the addition of a M42 armored mount. A new M54 mount with lighter, air-cooled Brownings resulted in the M15A1. A number of M15 Special vehicles were modified in Australia to carry a single Bofors 40-mm cannon. A further adaptation during the Korean was had the armored mount replaced by an automatic 40-mm gun and was designated M34 GMC.