The Messerschmitt Bf 109 is a German World War II fighter aircraft that was, along with the Focke-Wulf Fw 190, the backbone of the Luftwaffe's fighter force.^[​3] The Bf 109 first saw operational service in 1937 during the Spanish Civil War and was still in service at the end of World War II in 1945.^[​3] It was one of the most advanced fighters when it first appeared, with an all-metal monocoque construction, a closed canopy, and retractable landing gear. It was powered by a liquid-cooled, inverted-V12 aero engine.^[​4] It was called the Me 109 by Allied aircrew and some German aces, even though this was not the official German designation.^[​5]