Although several motorcycle companies experimented with horizontally opposed engines, only a few stuck with the layout for any length of time. BMW is the obvious one that comes to mind as it is a favorite on this site and in the garage. However, another marque also championed the layout for decades with some success in both sales and racing. They pre-dated BMW using the layout and were said to have influenced the famous boxer design that is still in use. That marque is Douglas. I was fortunate to see a few great examples of this marque when I visited the Solvang Motorcycle Museum, and vowed then to learn a little more about them given their similarity in engine layout to BMWs.
The Douglas brothers of Bristol first produced motorcycles in 1907. As true innovators in the engine arena, they also tried singles, parallel twins, and a compact V4 ! However, in 1914 they decided to place the 350cc horizontally opposed twin engine lengthwise in the frame, creating their most popular and lasting design. In the 1920s, Douglas really came into their own with success in the Isle of Man TT (senior and sidecar), and in dirt track racing. Despite the sporting success, the height of the brand's fame and recognition was probably in the early 1930s when Robert Fulton traveled around the world on a Douglas. Shortly after that, financial troubles set in, and Douglas was acquired.
Displacement continued to increase with 500cc and 600cc models over time and like other companies, WW II halted production. Unlike WW I where Douglas was a key supplier of motorcycles, in WW II they made generators and other products. They struggled after the war, and eventually was relegated to producing Vespa scooters under license. In 1955 they produced the 350cc Dragonfly which was a good looking bike, but underpowered compared to the competition. Douglas motorcycles disappeared a few years later.
Over their lifetime, Douglas displayed a continuing desire to experiment with layout and design, They produced bikes which used belt drive, and shaft drive as well as traditional chain. They produced singles, parallel twins, and horizontally opposed twins (in both lengthwise and crosswise configurations). They produced civilian and military bikes. They were competitive on both paved roads and dirt. However, they were consistent in their struggles with finances, and went from private to public and through a few different owners until their demise. Although the eventual outcome was not the best, this seems to have been one company where the engineers ran the show.