The Ordnance Corps decided to try and make a fast tank destroyer using a Christie suspension, a 37mm gun and a Wright/Continental R-975 angine. Although the original designs called fo a 37mm gun, it was upgraded to a 57mm, then a 75mm, and finally, the final design had a 76mm main gun. Also, the Christie suspension was replaced by a torsion bar suspension.
The M18 had several features allowing it to be maintained easily; the engine and transmission were both on steel rollers and could be disconnected from the rest of the tank destroyer and inspected for damage.
The prototype first saw action in Italy and production variants were used in North-West Europe and Italy.
It was designed form the beginning as a fast tank with a good engine and was very light so it could go at 60mph, which made it a possible way to counter the Maus; it would have been hard for the Maus to turn its turret fast enough to return fire, but the M18 could easily run out of fuel because the Maus had insane armour.
- 105 mm Howitzer Motor Carriage T88: M18 with the 76 mm gun replaced with a 105 mm T12 howitzer; canceled after the end of the war.
- 90 mm Cannon Motor Gun Carriage : M18 with the 76 mm gun replaced with a 90 mm Cannon; canceled after the end of the war/
- 76 mm Gun Motor Carriage T86 (Amphibious): M18 with a specially-designed flotation hull, using its tracks for water propulsion.
- 76 mm Gun Motor Carriage T86E1 (Amphibious): Same as T86, but with the addition of propellers for propulsion.
- 105 mm Howitzer Motor Carriage T87 (Amphibious): This model had the 105 mm T12 howitzer of the T88, and like the T86, used its tracks for water propulsion.
All work on the three amphibious models was canceled after the end of the war.