[2][3] In 1916 The Motor Cycle magazine claimed that the 1904 Fée was the earliest flat-twin motorcycle engine, of which there had since been many copies.

[1][2] From 1907 a 350 cc Douglas version was on sale, similar to the Fairy with the engine in-line mounted high in the frame, but without the chain driven countershaft beneath, and with belt final drive.

At the 1907 Stanley Show they attracted a lot of attention with a V4 engined motorcycle, with automatic inlet valves, and 2-speed drive.

One of the significant differences with this larger engine was the oil was carried in the sump and supplied by pump to bearings and cylinders.

In the 1920s Douglas built the first disc brakes, and had a Royal Warrant for the supply of motorcycles to the Princes, Albert and Henry.

In 1948, not long after the war, Douglas was in difficulty again and reduced its output to the 350 cc flat twin models.

The first of these models designated the T35 was one of the first production motorcycles to be fitted with rear suspension (swinging arm) unique in that the springing medium was a longitudinal torsion bar.

A version of Joseph Barter's horizontal twin cylinder engine of 1070 cc capacity, water cooled, was fitted to a two-seat cyclecar in 1913.

It was better equipped than the average cyclecar of the era featuring shaft drive from the front-mounted engine to the rear wheels and was sold for £200.

Production was suspended during World War I and when the car re-appeared in 1919 the engine was enlarged to 1224 cc and the price had risen to £400 then to £500.

Later in 1923 Jim Whalley won the French Grand Prix, a distance of 288 miles (463 km), and another Douglas won the 1923 Durban-Johannesberg Marathon race; a remarkable achievement by Percy Flook on a 2.75 hp machine with an average 43 mph (69 km/h) for 430 miles (690 km).

[16] The late twenties saw success in Austria (1929 Austrian TT was won by Rudolph Runtsch) Post war the factory had little road racing success however a Mark 3 did win an outstanding victory in the 1950 Bemsee 'Silverstone Saturday' beating all the Velocettes, Nortons and BSA Gold Stars Douglas's traditional layout of a flat-twin engine mounted with its cylinders parallel to the frame had been copied by several motorcycle manufacturers in the 1910s and 1920s.