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).