Three prototypes were built and ran before the end of 1946, well-proportioned coupes with a distinctive Rover style. By 1947, when changes in car taxation and government pressure to adopt a one-model policy conspired against it, the M-type seemed unlikely to be sold in sufficient numbers to fill the vast factory space acquired during the war. When it was abandoned, it left a gap in Rover’s future model strategy.
1947 Rover technical director Maurice Fernand Carey Wilks, Spencer’s younger brother, was impressed by a war-surplus Jeep he used at his holiday cottage in Anglesey, and for ground clearance at his home Blackdown Manor near Leamington Spa. A notable vehicle of the Second World War, the Jeep had been the result of United States Army Quartermaster Corps General EB Gregory’s specification for a quarter-ton combat truck, with a four-wheel drive capable of carrying three men and a.30 caliber machine gun. It had to bear a 600lb (272kg) payload and weigh no more than 1300lb (590kg).
All but a handful of the 135 American car manufacturers approached decided it was impossible. The deadline was critical. It left no time for elaborate engineering, only two of them had any hope of meeting it, the near-bankrupt American Bantam Company of Butler, Pa. that had made Austin Sevens under license, and Willys-Overland of Toledo, Ohio. Over three days in July 1940 engineer Karl Probst, who owing to Bantam’s precarious finances did not know if he would be paid for the work, designed a light truck with a wheelbase of 80in (203.2cm), a track of 47in (119.4cm), and light enough to be manhandled. The specification stipulated an incline approach angle (the slope it could climb without fouling) of 45 degrees, and a departure angle (a bank it could descend) of 35 degrees, together with the ability to wade through water 18in (45.7cm) deep. A prototype had to be available in 49 days.
Bantam managed it and Willys did not, but the small Bantam firm did not have the production capacity the army needed, so Willys was instructed to get on with development. Bantam’s engine had been rather feeble, and one reason Willys received the go-ahead was its engine, based on a 1927 Overland-Whippet side-valve 4-cylinder that had provided a meek 30bhp (22.4kW). It had been developed by Delmar G “Barney” Roos, who raised the compression ratio from 5.7 to 6.48:1, fitted aluminum pistons instead of cast iron, Cleveland graphite bearings, and manganese valve springs, and a counterweighted crankshaft. This raised the power to 60bhp (44.7kW) @ 4000rpm and torque to 100lbft (134Nm) @ 2000rpm.
To demonstrate the reliability of his amended engine Roos ran test examples flat out at 4400rpm continuously for 100 hours, proudly calling it the Go-Devil. The resulting Willys Model MB went into production, served on every battlefront with almost every Allied army; even the Wehrmacht cherished captured Jeeps. The lusty Go-Devil 2.2-liter produced sufficient power for nearly 60mph (96.3kph) and provided vigorous performance across the rough country. Willys made 362,841 of them,
and Ford joined in the programme to make another 281,446 calling it the GPW, identical save for the front chassis cross-member being an inverted U-section, not tubular. The Jeep was the archetype for millions of 4×4 utility and sports utility vehicles (SUVs) for the leisure and recreation markets that were to follow in the years to come.