In early-1943, the White Motor Company began development of a new prototype mortar carrier called the T19, which was based on the M3 halftrack personnel carrier. It was later accepted into U.S. Army service with the designation M21. The vehicle was powered by a 127hp inline 6-cylinder gasoline engine which gave it a maximum on-road speed of 72km/h. It was equipped with the 81mm M1 mortar as well as 12.7mm Browning M2 machine gun and an M1 or M9 anti-tank rocket launcher. During WWII, M21s provided close infantry support and continued in service until replaced by the M125 mortar carrier.