The Messerschmitt Me 210 was a German heavy fighter and ground-attack aircraft of World War II, designed to replace the Bf 110; design started before the opening of World War II. The first examples of the Me 210 were ready in 1939, but they proved to have unacceptably poor flight characteristics from serious wing planform and fuselage design flaws. A large-scale operational testing program throughout 1941 and early 1942 did not cure the aircraft's problems. The design entered limited service in 1943, but was almost immediately replaced by the Messerschmitt Me 410 Hornisse ("Hornet"). The Me 410 was a further development of the Me 210, renamed so as to avoid the 210's notoriety. The failure of the Me 210's development program meant that the Luftwaffe was forced to continue fielding the outdated Bf 110, despite mounting losses.