With the conversion of M3 Lee Medium Tanks to the successful 155mm-armed M12 Gun Motor Carriage, work began on an improved type based on the better M4 Sherman chassis. However, the M3 modifications utilized existing stocks of the old French-based M1917 artillery guns which were well beyond their prime and expectedly in short supply. As such, the 155mm M1 "Long Tom" rifles were mated to the Sherman M4A3 model chassis to create the "M40 Gun Motor Carriage" - known in its official U.S. Army longform as "155mm Gun Motor Carriage M40". While much of the components of the Sherman M4A3 were retained, the hull was widened some to compensate for the massive gun and mounting hardware.
The M40 appeared in pilot form as the "T83" prototype. Upon successful completion, the vehicle was adopted into service as the M40. However, it was not shipped in suitable numbers to Europe for combat use - in fact only a single prototype was used operationally. Production from the Pressed Steel Car Company did yield some 311 examples (some state up to 420) - the first in February of 1945 - but these arrived too late for service in the conflict and the large drawdown of American military goods stopped manufacture of the complete 600-strong stock originally envisioned. The M40 was not standardized in U.S. Army service until May of 1945.
Nevertheless, the M40 replaced the outgoing M12 models and these also entered service with the British Army (as the "155mm SP, M40 Cardinal"). It was available in number by the time of the Korean War (1950-1953) where it provided good service for ground forces.
As completed, the M4 Sherman origins of the M40 could clearly be identified, particularly at the sloping glacis plate at front. Running gear included six road wheels to a hull side with the drive sprocket at front and track idler at rear. The crew numbered eight and included the driver, commander, and up to six of the gunnery crew. All told, the vehicle weighed some 40 tons with 12mm of armor protection and its 155mm main gun barrel. Dimensions included a length of 29 feet, 10 inches, a width of 10 feet, 4 inches and a height of 8 feet, 10 inches. Power was served through a Continental R975 EC2 gasoline engine of 340 horsepower. This provided the vehicle with a maximum road speed of 24 miles per hour and a range of 105 miles. Suspension was through the same Horizontal Volute Spring Suspension (HVSS) arrangement seen in the Sherman M4A3 model.
The 155mm main gun fired a 95lb explosive shell out to 25,175 yards. It sat atop a complex recoil mechanism and structure that allowed for the necessary elevation. Onboard storage allowed for twenty of the massive 155mm shells to be carried so the M4A1 tractor served to tow an M23 ammunition trailer.
The T89 Howitzer Motor Carriage (detailed elsewhere on this site) was a related M40 development though mounting an even more powerful 203mm howitzer system. The vehicle was standardized in November of 1945 - missing out on action in World War 2 - and was designated as the M43 Howitzer Motor Carriage in service. These chassis was convertible back to M40 forms if needed.