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. Two pilot vehicles were to be built. 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. Though it weighed about 20 tons, the Hellcat was capable of traveling 55 mph. Its power came from Wright R-975, a nine-cylinder, 350-to-400-horsepower radial aircraft engine, paired to a 900T Torqmatic automatic transmission.