a battleship. He objected to the 3 inch M10 Gun Motor Carriage because it was too heavy and slow for his needs, and later on to the 90 mm M36 Gun Motor Carriage because it was essentially an M10 with a bigger gun. The United States Ordnance Department made several failed attempts to provide said vehicle using the weapons (the 37 mm, 57 mm, 3 inch, 75 mm and finally the lightweight 76 mm of 1942–1943) and technology available, including mounting the 3-inch gun on the fast M3 Light Tank chassis. The M18 was the end product of a long line of research vehicles aimed at providing the desired machine. In December 1941, the Ordnance Department issued a requirement for the design of a fast tank destroyer using a Christie suspension, a Wright-Continental R-975 radial aircraft engine, and a 37 mm gun on the chassis of the Light Airborne Tank T9 as the 37 mm Gun Motor Carriage T42. In April 1942, the Army requested that the 37 mm gun be replaced with a 57 mm and a coil spring suspension be substituted for the Christie suspension as the 57 mm Gun Motor Carriage T49, and authorized the construction of two pilot models. The Tank Destroyer Force, meanwhile, concluded that the same 75 mm gun M3 as used on the M4 Sherman medium tank would be better for a tank destroyer. The second pilot T49 was built with the 75 mm gun in an open-topped turret as the 75 mm Gun Motor Carriage T67 and was delivered in November 1942. After testing, it was revealed that a more powerful engine was necessary, and the 76 mm gun M1A1 being developed for the M4 Sherman had also impressed the Tank Destroyer Force. The T67 project was closed in January 1943, and the Ordnance Department directed that six pilot models of the 76 mm Gun Motor Carriage T70 be built with torsion bar suspensions, more powerful engines, new turrets and minor changes to the hull front. What became the M18 originated in Harley Earl's design studio, part of the Buick Motor Division of General Motors. Previously, basic designs for other kinds of vehicles had mostly originated from within the Ordnance Department. Buick's engineers used a torsion bar suspension that provided a steady ride. The first pilot was delivered in April 1943, and all six were completed by July. Once developed, the Hellcat