The M40 GMC was the first of a planned family of self-propelled artillery equipped with 155-mm guns or larger. It was intended to replace the M12 with a chassis based around the M4A3 tank which was modified to feature a Continental R975 engine as well as a horizontal volute spring suspension (HVSS) which was used on some late-war M4 variants. The M40 was equipped with the M2 (M59) 155-mm 'Long Tom' field gun which had superior range to the similar caliber M1918 gun used in the M12. Aside from that, the design shared many similarites with the M12, notably the minimally protected open top, lack of defensive armament, and use of a recoil spade in the rear to improve stability. The M43 HMC was virtually identical but was armed with the even heavier M1 (M115) 8-in howitzer in an interchangeable mount. Only one (pilot) M40 vehicle managed to see action during World War II with the 991st Field Artillery Battallion during the assault on Cologne, but both M40 and M43s were deployed during the Korean War. Aside from the US Army, its only foreign user was the British Army post-war where it was known as the Cardinal.

The prototype T83 entered production as the 155mm Gun Motor Carriage M40 in early 1945 but came too late to see action aside from a single vehicle. The 8" Howitzer Motor Carriage M43 was the only other variant built, in relatively small numbers, half of which were M40 conversions. A prototype 250-mm T94 mortar carrier was built but did not enter production, nor did a T30 cargo carrier. British versions were known as the Cardinal although unlike the earlier Priest, the name was never employed in US service.