One of the earliest production planes, the AVRO 504, isn't talked about all that much. WWI aircraft stories usually mention the Sopwith Pup and Camel, the Fokker Albatross and DR 1 (Triplane), the Nieuport Bebe, Spad VII or the SE5A.
However, the 504, far from being insignificant, held a few notable achievements. The first being, it was built by AVRO, which itself was the world's first incorporated aircraft manufacturer. After developing various new designs and refinements, as well as claiming many aerial firsts in Britain, A. V. Roe designed the 504 as an improvement on earlier 500 series aircraft and flew it first in 1913. Claiming attention from the Royal Flying Corps (there was no British Air Force yet), it was put into production for them. During WWI, it claimed the first bombing attacks against Germany and the first strafing attacks against ground troops. It was also the first aircraft shot down with anti-aircraft fire on the allied side.
By TSRL - Own work, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=7050881
As the war progressed, newer designs, more agile and faster soon took its place as a combat aircraft, but it stayed in production with thousands being built as trainers. By the end of the war, over 8,300 of them were built.
One of the first armed aircraft, they were the first to use Lewis guns as shown on this night fighter version.
However, the end of the war didn't mean the end of the 504. It continued on as a trainer, with production going right up to 1933, an impressive feat for such an early design. Not only was Britain using the type, but Russia had a few and built their own copies as well, the Avrushka U-1(little Avro). In fact, a quick look at Wikipedia makes it look like much of the world used this trainer, with well over thirty different countries (including the USA) putting them to use. The whole list can be found here:
https://en.wikipedia.org/wiki/Avro_504
There's also a lengthy list of a couple dozen different manufacturers around the world who built the plane under license.
Even after they went out of production, there were still many in service, some even during WWII for use in pulling gliders and other non fighting service.
Including foreign production, over 11,300 were eventually built.
This shot shows the first airplane to land in Iceland in 1919...it was an AVRO 504.
So if you're watching an old WWI movie, or see a picture or painting of a biplane with a single skid between the wheels, chances are you're looking at a 504.