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 the project was cancelled 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.