BSA, a company that began as an armament's manufacturer supplying the armed forces, had geared up to produce munitions as early as 1935. Since the late 1920s the company had also presented the War Office with a number of motorcycles for evaluation. The War Office then issued a new specification for a lightweight model, specially designed for service use, and BSA, along with the other major manufacturers submitted their own prototype, the M20, for service trials. This was well received, and a few had been built when, with war imminent, the official policy changed to favor machines that were already in service. Those companies whose reliability was well known (principally the Norton 16H and BSA M20) received large contracts which eventually led to the M20 becoming the leading service motorcycle employed by the British forces during the Second World War - although it was far from the most popular with its riders. The M20, designed in 1937, as a sidecar model (which overloaded it) utilized a 500cc air-cooled single-cylinder side-valve engine in a heavy frame. Heavy, bulky, slow and with limited ground clearance, the M20 had far from ideal specification, but it was rugged, generally reliable and easily repairable. Special fittings included a long, spiked prop stand for field use and a large headlamp, fitted with a blackout mask in many areas of operation.