Designed by Willy Messerschmitt and Robert Lusser in response to an RLM 1934 interceptor requirement, the Bf 109 V1 first flew on 29 May 1935—powered by a borrowed Rolls-Royce Kestrel because German engines weren't ready. It was the world's most advanced fighter: all-metal monocoque, closed canopy, retractable gear, and Handley Page leading-edge slats. Earlier Bf 109 variants (notably the B and C) fought the Spanish Civil War; the "Emil" became the main Luftwaffe fighter for the Battle of France and the opening rounds of the Battle of Britain. Over 33,000 were built across variants from the early Jumo-powered models to the late-war "Kurfürst," making it the most produced fighter in history. Pilots either loved its responsiveness or cursed its narrow landing gear and high-speed handling—there was no middle ground.