The Messerschmitt Bf 109, was a German World War II fighter aircraft that was the backbone of the Luftwaffe's fighter force. The BF 109 first saw operational service during the Spanish Civil War (1939) and was still in service at the dawn of the jet age at the end of World War II (1945). It was one of the most advanced fighters of the era, including such features as all-metal monocoque construction, a closed canopy, and retractable landing gear. It was powered by a liquid-cooled, inverted-V12 aero engine. From the end of 1941, the BF 109 was steadily being supplemented by the superior Focke-Wulf FW 190.