Development started in the spring of 1944 due to a shortage of M1918 guns for the M12. The vehicle was based on the M4 Sherman and featured a HVSS-type suspension. The design allowed the 155-mm M1 gun and 203-mm M1 howitzer to be interchangeable (an SPG with the latter was designated the M43). The vehicle first saw service during Operation Zebra. A total of 418 M40 and 48 M43 vehicles were manufactured (another 24 M40s were reequipped).
M40
Wright Continental R-975C1
SCR 510
155 mm Gun M1A1
M43
Wright Continental R-975C4
SCR 610
8-inch Howitzer M1
Ford GAA early
SCR 619
Characteristics are specified for vehicles with crews trained to 100%.