Born in Schrimm in the Province of Posen, Wilcke volunteered for military service in the "Reichswehr" of the Third Reich in 1934. Initially serving in the "Heer" (Army), he transferred to the Luftwaffe (Air Force) in 1935. Following flight training, he was posted to "Jagdgeschwader" "Richthofen" (Fighter Wing "Richthofen") in April 1936. After an assignment as fighter pilot instructor he volunteered for service with the Condor Legion during the Spanish Civil War in early 1939. After his return from Spain, he was appointed "Staffelkapitän" (squadron leader) of the 7. "Staffel" (7th squadron) of "Jagdgeschwader" 53 (JG 53—53rd Fighter Wing). Following the outbreak of World War II, he claimed his first aerial victory on 7 November 1939. On 18 May 1940, during the Battle of France, he was shot down and taken prisoner of war. After the armistice with France, he returned from captivity and was appointed "Gruppenkommandeur" (group commander) of the III. "Gruppe" (3rd group) of JG 53 during the Battle of Britain, claiming 10 victories over England.