The BSA B31 is a motorcycle that was produced by Birmingham Small Arms Company.
The BSA B31, introduced in 1945, was the first new model introduced by the company after the Second World War. Based on pre-war designs, it used a single cylinder four stroke engine that displaced 348 cc (21.2 cu in). Initially, it had a rigid frame and telescopic forks, the first use of such on a BSA. It developed about 17 bhp (13 kW), adequate for the roads of the day and enough to deliver a top speed of around 70 mph (110 km/h).There was a competition kit available for the original rigid model B31 and B33. It was immediately popular and was soon joined by a 500 cc (31 cu in) version, the BSA B33 and competition equivalents, the BSA B32 and BSA B34; the side-car versions, with stiffer suspension and different final demultiplication, were known as BB31 and BB33.
The core engine was acquired in the UK. The parts needed to make it a complete engine came from Australia, Canada, Japan, the UK, and the USA. Other miscellaneous parts were sourced in countries not mentioned above.