William and Edward Douglas founded the Douglas Engineering Co in Bristol in 1882 as a blacksmith then foundry. They acquired W J Barter from the receiver and inherited he firm's flat twin engine design. From 1907 a 350 cc Douglas version was on sale. In 1915 the engine was placed lengthways in the frame with belt final drive, and electric lighting and during the Great War,  Douglas became a major supplier, making around 7,000 motorcycles for the army.

Post WW I, Douglas motorcycles became popular in dirt track racing. The 1923 RA model with disc brakes was favoured initially and this prompted Douglas to build specific dirt track models. These motorcycle designs were gradually increased in size and power with 500 cc and 600 cc engines fitted to the DT5 and DT6 Dirt Track models in the late 1920s and early 1930s. The engines had hemispherical heads and a short rigid forged crankshaft. They dominated dirt track racing for about three years. In 1929, the most successful dirt racing year, 1,200 Dirt Track motorcycles were sold.

Inevitably the DT engine came to the attention of the fledgling 500 constructors adopted this power plant including: Jim Bosisto's Buzzie, Ken Neve's KN, Adrian Butler and Bruce Mardon's Stromboli, Trevellick and probably the Australian Rowe and Warburton Special.

In reality, this 1930s engine, while readily and cheaply available,  normally only produced 27-30 hp and even extreme tuning would not take this past 35 hp, about 10 short of the JAP with the inevitable outcome.

A per pro nothing in particular, Fay Taylor, Brooklands racer and Nazi, was a regular Douglas Dirt Track rider....