Avro 527

The Avro 527 was two-seat fighter which was the last derivative of the 504 design. It was built for the RFC and was ready to fly in early 1916.
It was powered by a 150hp Sunbeam Nubian eight-cylinder engine water-cooled engine. It copied both mainplanes and tail assembly from the 504, but the rest differed greatly from it.
It was going to be armed with a free-mounted 0.303in Lewis Machine Gun in the rear cockpit.
When tested, it showed a poor climb rate and the pilot's visibility was seriously reduced by the twin exhaust stacks of the engine and the radiator.
Only one prototype was built and, due to the uninterest of the RFC, development on it was discontinued during 1916. It was proposed a version with a longer wingspan, the 527A, but there aren't plausible records of it being matherialized.

Sources:
1. https://en.wikipedia.org/wiki/Avro_527
2. Salamander Books - The Complete Book of Fighters