The M15 was designed to improve upon the firepower of existing American anti-aircraft vehicle and protect American ground forces entering into combat in North Africa in 1942. To do this, it had the same two 12.7 mm Browning M2 Machine Guns commonly used in other American half-tracks, but coupled them with a 37 mm M1A2 Autocannon in an armored compartment. The added punch of the 37 mm was more than enough to down any low-flying aircraft if a hit should occur.
Based on the M3 Half-Track, the M15 used the same White 160AX Engine which was capable of propelling the vehicle at speeds of up to 72 km/h on roadways.
The M15 began its development through the T28 project, a series of tests on the feasibility of the M42 Combination Mount on an M2 Half-Track. Tests showed that the 37 mm autocannon put too much stress on the chassis and the project was ended in April 1941. With the invasion of North Africa coming in 1942, the need for an anti-aircraft vehicle to protect American ground forces became critical and thus the project was restarted, now using the M3 Half-Track as a base. The new prototype was designated the T28E1, but with no time to properly test the vehicle, the T28E1 was quickly redesignated the M15 Combination Gun Motor Carriage and pushed into early production with 80 being built between July and August, 1942.
In North Africa, the vehicle became a huge success and earned a reputation for being an effective anti-aircraft weapon. Furthermore, AA crews liked the vehicle for the added protection it provided with its armored firing compartment.
|“||The proficiency of this mobile weapon can be attributed to three characteristics: its mobility, enabling it to work well in close support of combat troops in forward areas and to patrol roads over which heavy traffic must travel under constant threat of bombing and strafing; its flexible firepower, combining the volume of caliber .50 with the knocking power of the 37-mm and the facility which its fire is controlled, by using the tracer stream of one caliber .50 to bring it on the target before opening with the full volume of its armament. Numerous cases are cited in which a 'mouse trap' effect has been obtained when enemy planes came in much closer on the initial caliber .50 fire than they would on light canon and are caught by the 37-mm - Official US Army Report on M15 in North Africa||”|
From North Africa, the M15 continued service with United States forces through the Mediterranean as well as from Normandy through Europe and in the Pacific. In fact, M15s were so common that alongside their counterpart, the M16 Multiple Gun Motor Carriage, each US Army armored division was equipped with eight vehicles.