In 1952, Leyland began experimenting with ideas for a rear-engined double-decker bus. A prototype was built, with a body by Saunders-Roe, to the maximum permitted width of . It was fitted with a turbocharged version of the Leyland O.350 engine, which was transversely mounted at the rear of the sub-frame. The chassis was a platform-type frame of steel and light alloy with deep stressed side-members. An automatic clutch and self change gearbox were also fitted. The vehicle was designated the PDR1 (R for "Rear-engined").