He teamed up with William Douglas (of Douglas Motorcycles to develop new prototype motorcycles under the name Williamson-Douglas and employed Billy's brother Harold as a test rider.

Douglas had been developing a 964 cc water-cooled flat twin engine that could be used either for light cars or motorcycles.

Billy Williamson fitted this engine into a frame with Douglas-Druid girder forks and a Douglas two-speed gearbox and a foot-operated clutch which was launched in 1912 at a cost of £82.

Production was halted by World War I and in 1919 the only engines available were JAP 980 cc air-cooled side valves, so Williamson redesigned the frame to fit.

Unfortunately Billy Williamson suffered a fatal heart attack in 1920 after only twenty motorcycles had been produced.