The T87 105mm Howitzer Motor Carriage (Amphibious) was part of a programme to develop amphibious versions of the M18 Hellcat tank destroyer for use in the Pacific.
Work on the T86 76mm Gun Motor Carriage (Amphibious) began early in 1944. Harmon-Herrington was given a contract to produce three prototypes. The T86 and T86E1 were armed with the same 76mm gun that was used in the M18 while the T87 was to get the 105mm howitzer that was also used in the T88 105mm Howitzer Motor Carriage (also based on the M18).
The T86 and T86E1 retained the lower hull and turret of the standard M18, but had a new upper hull (between the top of the tracks and the turret). This was a light-weight floatation hull with a straight-edged bow. The T86 was to be propelled in the water by its tracks while the T86E1 had two propellers. The T87 was to use whichever system proved to be superior.
The T86 and T86E1 were tested in the spring of 1944 and the track propulsion system was preferred. The T87 was thus built to use that system, although with a modified track. The hull was also altered, giving it more rounded corners and shortening it by 2ft 3in. It carried the 105mm howitzer in an open topped turret and was lighter than the T86E1.
The T87 began testing in December 1944, but it was still undergoing trials when the war ended. Both the T87 and T88 projects were then cancelled.