|Date of first acceptance||January 1944||Total acceptances||110|
|Manufacturer||White Motor Co.||Crew||6 men|
|Length with winch||244.875"
|Width over mine racks||87.5"
|81mm Mortar M1||Mount M1||97 rounds||60°
(30° left and right;
|+85° to +40°
|.50cal M2HB MG||Flexible on pedestal mount||400 rounds||360°
|Rolled face-hardened steel|
|Location||Thickness||Angle from vertical|
|Engine||White 160AX; 6 cylinder, 4 cycle, in-line gasoline|
|Horsepower||Net: 147@3000rpm||Torque||Net: 325 ft-lb@1200rpm||Fuel capacity||60gal
|Transmission||Spicer 3461 constant mesh, 4 speeds forward, 1 reverse|
|Type||Road wheels||Track return rollers|
|Front: Semi-elliptic longitudinal leaf spring
Rear: Vertical volute spring
|Front: Steel ventilated disc
Rear: 1 bogie/track;
|Drive sprockets||Idlers||Shock absorbers||18-tooth front drive||Spring-loaded at rear of track||On front wheels|
|Center guide band type|
|Pitches/track||58||Track ground contact length||46.75"
|Max level road speed||45mph
|Max vertical obstacle||12"
|Min turning diameter||59'
|Max fording depth||32"
|Cruising range||~200mi, roads
The M21 mortar carrier was based on the half-track M3 and was similar to the M4 mortar carrier. In the M21, however, the mortar was fired to the front, and so the .50cal MG was positioned in the very rear of the vehicle, just in front of the rear door. The M21 also had rod-type stowage racks on the sides of the vehicle, which resembled ladders in construction. The top rod was used to secure the passenger compartment's canvas cover, and the bottom rod was used to secure stowage.