When the United States committed its vast resources and manufacturing prowess to world war in December of 1941, it found itself with lacking in war-winning goods of all kinds - small arms, aircraft, warships, combat vehicles and the like. 1942 therefore gave rise to many internal programs in an effort to develop weapons of war to help counter the firepower of the Axis forces in North Africa, the Pacific and in Europe. One such project became the "M12 Gun Motor Carriage", a tracked, self-propelled gun (SPG) platform mounting a massive 155mm gun barrel and mated to the chassis of the existing M3 General Grant/General Lee Medium Tank.
The M3 Medium Tank (forerunner to the more famous M4 Sherman series) was designed throughout the latter half of 1940, ultimately entering production in August of 1941 and wrapping up in December of 1942 to the tune of 6,258 vehicles delivered. The type was a functional beast for its time, mounting a capable 75mm main gun alongside a turreted 37mm cannon while being defensed by up to four .30 caliber machine guns. Power was served through a Continental Motors R975 EC2 engine of 400 horsepower (a Wright R-975 Whirlwind built under license) mated to a synchromesh transmission system. Maximum road speed was 26 miles per hour across ideal surfaces while range fell short of 120 miles.
In practice, the M3 proved serviceable for the burgeoning American war effort and saw extensive combat service at the hands of several nations beyond the United States including Australia, Canada, New Zealand, the United Kingdom and the Soviet Union (Lend-Lease ensured it found its way across both oceans). However, the design lacked in several key areas, principally in the mounting of its 75mm main gun in a limited-traverse side sponson, most times requiring the entire vehicle to be turned into the face of the enemy when engaging - a cumbersome and time-consuming process to say the least. Additionally, the M3 proved slow when going cross-country and its high profile (due to its compound turret design) made it cannon fodder to enemy gunners at range along the horizon. As such, the M3 fell to history with the arrival of the M4 when numbers allowed.
Regardless, the available and proven chassis of the M3 was now selected to serve in other battlefield roles, making it a logistically-friendly war time solution since automotive parts could be shared along production lines as well as when undergoing repairs in-the-field. The M3's basic hull design was completely rewritten for the role, incorporating an all-new superstructure intended to protect against small arms fire and artillery spray. However, only the driver was completely encased in steel armor while the gunnery crew resided in a rear, open-air compartment with the main gun armament (necessitated by the large gun breech and room to maneuver the large 155mm shells). A canvas tarp could be affixed across several support arms over the breech but little else protected the gunnery crew form the dangers of the modern battlefield. The crew of six included the driver, vehicle commander, several gunners and several ammunition handlers. A dozer blade was added to the rear of the hull and lowered when firing to help counter the inherently violent recoil effects of the main gun.
Key to the design of the new SPG gun platform was the selection of the M1917 series 155mm field gun. The M1917 was a proven commodity and the US Army held stocks of the weapon from its days participating in World War 1. The M1917 was essentially the French 155mm GPF which gave excellent service during its heyday in the Great War and it made logistical sense once again to couple an existing weapon with an existing chassis. The gun was available in the "M1917" and "M1917A1" flavors and also appeared in an "M1918 M1" form. Depending on existing gun stocks of these gun barrels, the new SPG vehicle could make use of any three as available - all firing a large 155mm high-explosive shell. The recoil mounting mechanism supported the weapon atop the chassis just aft of the vehicle's center. While traversal was limited, elevation was possible to an extent. Born as an indirect-fire field gun (and not a direct-fire weapon such as an anti-tank gun), the M1917/M1918 series guns were proven weapons at lobbing explosive devices against target areas. Self-defense was through a single .50 caliber Browning M2 heavy machine gun - suitable for engaging light-armored vehicles, infantry and low-flying aircraft. Since such SPG platforms would be operating behind the main line of advance, this defense was deemed sufficient. Additionally, any personal weapons carried by the crew could be brought to bear. The new SPG vehicle was designated in the US Army inventory as the "M12".
The M12 weighed in at 26 tons and was powered by a Continental R975 EC2 gasoline engine as in the M3 Grant/Lee series tanks. Maximum speed was approximately 23 miles per hour on ideal surfaces with an operational range of 135 miles though performance dropped off considerably when off-road. The chassis sat atop a Vertical Volute Suspension System (VVSS) common to American tracked vehicles of the period and this automotive arrangement allowed the vehicle to keep pace with other mechanized army forces as needed.
In service, M12 production was extremely limited for more capable SPG solutions arrived in time. As such, approximately 100 vehicles were only ever produced and many were initially utilized for training of new gunnery, driver and command crews in the fine art of battlefield management while other systems went unused in storage. However, for the mid-1944 D-Day landings in Northern France, some 74 M12 vehicles were prepared for action with additional equipment and these systems ultimately found their way into Europe with the advancing US Army on their march across France. The M12 proved its worth in engaging specific target areas with their 155mm ordnance while also being called upon to lay waste to complete structures occupied by the enemy. By this time, the "M30 Cargo Carrier" - a similar M3 Grant/Lee-minded conversion though lacking the 155mm artillery gun - was fielded alongside M12 gun carriers as dedicated ammunition carriers for the M12 hull was limited to carrying just 10 x 155mm projectiles aboard. The M30 ammunition carrier added a further 40 x 155mm projectiles to each pair of M12 gun systems fielded.
Only a single M12 example survives today (2013), this under the care of Fort Sill in the United States. The M12 was given the formal US Army designation of "155mm Gun Motor Carriage" in keeping with US Army military nomenclature of the time. Similarly, the M30 carrier was known as the "Cargo Carrier M30".