The twin camshaft RM models were the work of Harry Rush, Riley's Chief Designer. This 1.5 litre car is only distinguishable from the 2.5 litre by its shorter bonnet and radiator badge in dark blue, as opposed to pale blue.

Among the first British cars to use independent front suspension and rack-and-pinion steering, the specification of the RM was otherwise conventional. The body is built on an ash frame, with a fabric over a wire-mesh roof.

The 2.5 litre RM, was among the fastest saloons of its day and attracted the discerning motorist like Lord Louis Mountbatten. Production of the RM also included about 500 each of the 3-seater roadster and drophead coupé models, designed for the American market.