The Messerschmitt Me 309 was a prototype German fighter, designed in the early years of World War II to replace the Bf 109. Although it had many advanced features, the Me 309's performance left much to be desired and it had so many problems that the project was canceled with only four prototypes built. The Me 309 was one of two failed Messerschmitt projects intended to replace the aging Bf 109, the other being the Me 209 of 1943.
The Me 309 project began in mid-1940, just as the Bf 109 was having its first encounters with the Spitfire in the Battle of Britain, the first aircraft to match the 109 in speed and performance. Already, Messerschmitt anticipated the need for an improved design to replace the Bf 109. The Reich Air Ministry, however, did not feel the same urgency, with the project given a low priority, resulting in the design not being finalized until the end of 1941.