The engine was designed at the Royal Aircraft Factory based on the 8-cylinder RAF 1 model. A notable feature of the engine was the control of both intake and exhaust valves by a single shared tappet.
Production was undertaken by Armstrong Whitworth and D. Napier & Son Ltd. The RAF 3a was the main production variant with an increased cylinder bore compared to the prototype.
A total of 260 units were produced at Napier & Son and 29 at the Armstrong-Whitworth factory. The engine was used in the Airco DH.4 light bomber aircraft.
| Configuration | 12-cylinder V-type 60° |
| Cooling | liquid |
| Compression ratio | 5.3 |
| Displacement | 17.2 l |
| Weight | 390 kg |
| Maximum power | 260 hp at 1750 rpm |
