he M21 Mortar Motor Carriage (MMC for short) was an American self-propelled mortar from the Second World War. The first prototypes were built in 1942, and serial production was carried out in the period 1944-1945. In total, about 100-120 copies of this vehicle were built. The M21 was powered by a single engine White 160AX with a power of 128 hp . It was armed with a single 81 mm M1 mortar and a single 12.7 mm M2 Browning machine gun.
The M21 Mortar Motor Carriage was developed at White Motor Company based on the chassis and hull of the M3 Half-Track Transporter. The M21 vehicle was developed to complement – later replace – the M4 and M4A1 self-propelled mortars. The main difference was the use of a different chassis and the location of the mortar in the combat compartment. The M21 had good off-road characteristics, good performance and sufficient armament to support the advancing own infantry with indirect fire at close range. Vehicles of this type took part in the declining period of World War II: primarily in the fights in France in 1944, in Belgium and in western Germany in 1944-1945. They also took part in repelling the German offensive in the Ardennes at the turn of 1944-1945.