After the success of the M12 155mm Gun Motor Carriage in Western Europe, the Army concluded by 1944 that a new self-propelled gun was needed because the supply of old 155mm guns used on the M12 had been exhausted. The new vehicle used the M4A3E8 Sherman tank chassis and could mount either the M1 155mm Gun or the M1 8-inch Howitzer. The M40 was the self-propelled version of the 155mm Long Tom. Its first prototype was produced in July 1944, and testing with both the gun and howitzer was very successful. Production began in early 1945, and the Pressed Steel Car Company manufactured 418 M40s before production ended in late 1945. Two prototypes saw service during the attack on Cologne, Germany, but no production M40s entered service during World War II. The M40 did see significant action during the Korean War, where it proved to be an effective and powerful weapon.