In December 1941, the Ordnance Corps issued a requirement for the design of a fast tank destroyer using Christie suspension, the Wright-Continental R-975 radial engine, and a 37 mm gun. Two pilot vehicles were to be built.
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 developed an innovative torsion bar suspension that provided a steady ride. Though it weighed about 20 tons, the Hellcat was capable of traveling upwards of 60 mph. Its power came from a nine-cylinder, 450-horsepower radial-type aircraft engine paired with a three-speed Hydramatic transmission.
Changes to the specification mean that the first pilot - the T49 GMC - was built with the British (57 mm) QF 6-pounder gun instead of the 37 mm and torsion suspension instead of Christie. 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 T67 GMC. 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 T70 GMC - were built with this gun. The trials of these led to a new turret and changes to the hull front but the design was otherwise accepted for production which began in mid-1943.
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 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 strategic testing of the Hellcat prototypes proved "superlative", 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, and replaced the fixed, rigid structure of other units with a much more flexible command structure that allowed adapting to more complicated tasks.
Production of the M18 Hellcat began in mid-1943 and ended in October 1944. The project was so secretive that a story about the “new” tank destroyer ran in newspapers just a month before production.