When the Tank Destroyer Force was organized in 1941, their commander Lieutenant Colonel (later General) Andrew Davis Bruce envisioned the units being equipped with something faster than a tank, with a better gun but less armor to allow for speed, a cruiser rather than a battleship.[7] He objected to the 3 inch M10 Gun Motor Carriage because it was too heavy and slow for his needs,[8] and later on to the 90 mm M36 Gun Motor Carriage because it was essentially an M10 with a bigger gun.[9] The United States Ordnance Corps 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.[10] The M18 was the end product of a long line of research vehicles aimed at providing the desired machine.
In December 1941, the Ordnance Corps issued a requirement for the design of a fast tank destroyer using a Christie suspension, the Wright-Continental R-975 radial engine, and a 37 mm gun. Two pilot vehicles were to be built.[11]
The M18 originated in the design studio of Harley Earl from the Buick motor company division of General Motors, whose team also worked extensively on early camouflage paint.
Buick engineers used a torsion bar suspension that provided a steady ride.[a] Though it weighed about 20 tons, the Hellcat was capable of traveling upwards of 60 mph. Its power came from Wright R-975, a nine-cylinder, 450-horsepower radial-type aircraft engine, paired with a three-speed 900T Torqmatic transmission.
Changes to the specification mean that the first pilot - the 57 mm Gun Motor Carriage T49 - was built with the British (57 mm) QF 6-pounder gun instead of the 37 mm and a torsion bar suspension instead of the Christie suspension. It was tested in 1942 but the army wanted a heavier gun - the same 75 mm gun M3 as used on the M4 Sherman medium tank. The T49 project was cancelled and the second pilot was built with the 75 mm gun as the 75 mm Gun Motor Carriage T67. This met approval but in early 1943 the army requested a more powerful gun - the 76 mm gun M1 under development for the Sherman. Six pilot models - as the 76 mm Gun Motor Carriage T70 - were built with this gun. The trials of these models led to a new turret and changes to the hull front but, the design was otherwise accepted for production which began in July 1943.[11]
Once developed, the Hellcat was tested in the same manner as passenger cars before and after it, at the General Motors Milford Proving Ground. Top speed testing was done on a paved, banked oval and ride quality tests were done over specially developed stretches of bumps. The M18 also required additional tests of its ability to ford six feet of water, climb small walls, and ram through structures.
The first models of the tank destroyer were tested by the US Army's 704th Tank Destroyer Battalion. The unit had originally been trained on the M3 Gun Motor Carriage (a 75 mm gun installed in the bed of an M3 half-track). Despite its T70 prototypes requiring several improvements, the 704th had "superlative" testing record, and the unit was later issued production Hellcats after many of their suggestions were integrated into the vehicle. The testing phase of the Hellcat proved that teamwork was an essential element of the new light tank destroyer units[citation needed], and replaced the fixed, rigid structure of other units with a much more flexible command structure that allowed adapting to more complicated tasks.[12]