The 105mm M7 self-propelled howitzer was first used by the Marines on Okinawa.
Under the G-series Table of Organization, each infantry regiment was equipped with four M7 HMC's in the regimental weapons company.
Plans for mounting a 105mm howitzer on the chassis of the M3 medium tank to provide self-propelled artillery support for armoured divisions were made in June 1941 as soon as the M3 was in production. Two pilot models were constructed, designated T32, based on M3 medium chassis, but with an open topped superstructure. The standard M1A2 105mm howitzer was installed with its carriage suitably modified to fit. The weapon was offset to the right of centre. The trials vehicles were successful and the design was standardised as the M7 HMC in February 1942. Changes (made in the T32) for production vehicles included modified front shields and a cupola and ring for an AA machine gun. American Loco started production in April and built 2028 in 1942. Late production vehicles had M4 type bogies with trailing return rollers; otherwise the M7 had identical chassis and mechanical specification to the M3 medium tank. A few late production M7s also had a one-piece cast nose instead of the original three-piece boIted type.
Meanwhile the M4 had superseded the M3 in production and in September 1943 it was proposed to continue M7 production on the chassis of the M4A3 medium tank. Designated M7B1, this vehicle differed from the M7 (aside from its different power plant) in having hinged side plates for added ammunition protection, a cast one-piece nose, and bogies with trailing return rollers. Pressed Steel Corporation built 826 from March 1944-February 1945, after which Federal Welder built another 127 vehicles of both M7 and M7B1 type by the war's end.
With the standardisation of the M37 HMC on the Light Combat Team (M24) chassis (qv) in January 1945, the M7 and M7B1 were reclassified to "substitute standard" and were gradually replaced by the M37 from then on. The M7 and M7B1 HMCs were standard equipment of artillery battalions in all American armoured divisions.
Special features/remarks: Very light gun for such a large chassis, but the M7 series were an expedient type got quickly into service and enjoying the advantages of standardisation and reliability inherent in the M3/M4 medium chassis. Proved very successful in service though crew protection was limited.