The M55 was the largest US Army mechanized artillery piece in service in the 1950s.
The rise of armored forces in World War II led to the need for artillery that could follow the tanks cross country. In its ultimate manifestation, this led to the post-war development of a tracked, fully enclosed, 8-inch howitzer for the US Army.
Pacific Car & Foundry (PACCAR) of Renton, Washington, had experience building armored vehicles in World War II and received the contract to build the T108, later called the M55, in the early 1950s. The vehicle portion of this weapons system was based on components from the M46, M47, and M48 medium tank family. However, the engine and drive sprockets were located in front of the turret on the M55.
Though lightly armored, the turret did provide protection against small arms fire and some shrapnel. It could also be fitted with attachments to limit exposure to nuclear fallout. The spade at the rear of the vehicle was buried when firing the howitzer. The crew used an onboard, mechanical loader/rammer for the heavy projectiles.
The US Marines continued to use the M55 in Vietnam even after it was phased-out by the Army.