 The T30 HMC was originally conceived in 1941 as an interim design to fulfil the Armored Force 's requirement for an assault gun to equip tank and armored reconnaissance units . The Ordnance Department design was based on the M3 Half @-@ track in order that it could be brought into service quickly . A prototype vehicle was authorized in October 1941 armed with an <unk> 75 mm Pack Howitzer and a mount that was designed to fit on a simple box structure in the back of an M3 Half @-@ track . 
