Willius was born on 5 November 1919 in Mainz-Kostheim. Following pilot training, "Obergefreiter" Willius was posted to 2. "Staffel" (2nd squadron) of "Jagdgeschwader" 20 (JG 20—20th Fighter Wing). At the time, this squadron was initially commanded by "Oberleutnant" Albrecht Freiherr von Minnigerode and then by "Oberleutnant" Fritz Stendel. The "Staffel" was subordinated to I. "Gruppe" (1st group) of JG 20 which was commanded by "Hauptmann" Hannes Trautloft. The "Gruppe" was equipped with the Messerschmitt Bf 109 E and fought in the Battle of France under the control of "Jagdgeschwader" 51 (JG 51—51st Fighter Wing). Following the Armistice of 22 June 1940, I. "Gruppe" of JG 20 was officially integrated into JG 51, becoming its III. "Gruppe" on 4 July. In consequence, Willius became a member of the 8. "Staffel".