He also fought in the Battle of Britain, Operation Barbarossa, the German invasion of the Soviet Union, and in Siege of Malta before again transferring to the Eastern Front in 1942.

In February 1944, he transferred to Jagdgeschwader 3 "Udet" (JG 3—3rd Fighter Wing), initially commanding the IV.

Müller, who was nicknamed "Tutti", was born on 25 December 1916 in Berlin-Lichterfelde, at the time in the Kingdom of Prussia of the German Empire.

[2] His flight training began on 1 March 1939, at first at Oldenburg, and on 17 July 1939 at the Jagdfliegerschule (fighter pilot school) at Werneuchen.

[2][Note 2] World War II in Europe began on Friday 1 September 1939 when German forces invaded Poland.

[4] Müller flew combat missions during the Phoney War period and received the Iron Cross 2nd Class (Eisernes Kreuz zweiter Klasse) on 17 April 1940.

Gruppe provided fighter cover in the vicinity of Compiègne during the armistice negotiations which was signed on 22 June.

[7] On 15 September, Müller ran out of fuel in his Messerschmitt Bf 109 E-4 (Werknummer 5251—factory number), resulting in a forced landing in the English Channel.

[8] In preparation of Operation Barbarossa, the German invasion of the Soviet Union, JG 53 arrived in Mannheim-Sandhofen on 8 June 1941 where the aircraft were given a maintenance overhaul.

[16] On 20 December, on a Junkers Ju 88 escort mission to La Valletta, Müller claimed a Hawker Hurricane shot down, his 21st aerial victory.

[18] In support of Case Blue, the 1942 strategic summer offensive in southern Russia, I. Gruppe of JG 53 began its relocation to the Eastern Front on 28 May 1942, arriving at Kursk that evening.

[18] Müller claimed his first aerial victory in this theater of operations on 3 June over an I-61, a designation for the Mikoyan-Gurevich MiG-1 fighter aircraft.

On 18 September 1942, flying a "gunboat" Bf 109, Müller claimed six Il-2 ground-attack aircraft, taking his total to 97 aerial victories.

Three other Luftwaffe officers were presented with the Oak Leaves that day by Hitler, Oberleutnant Wolfgang Tonne, Leutant Hans Beißwenger and Feldwebel Crinius.

[29] Following the presentation, Müller, Tonne and Crinius were ordered to Berlin where they made a propaganda appearance at the "House of the Press".

[30] At the same time, I. Gruppe of JG 53 relocated from the Eastern Front to Comiso Airfiled in Sicily where they arrived on 10 October.

[31] When British forces launched the Second Battle of El Alamein on 23 October, elements of I. Gruppe of JG 53 were ordered to North Africa.

[32] In November 1942, Müller was appointed Gruppenkommandeur (group commander) of I. Gruppe of JG 53 and led the unit in Tunisia.

[34] Müller claimed his first aerial victories in this theater of operations on 31 January 1943, shooting down a United States Army Air Forces (USAAF) Boeing B-17 Flying Fortress between Béja and Souk El Khemis.

[36] on 31 January 1943, he claimed his two aerial victories, shooting down a USAAF B-17 bomber and a Lockheed P-38 Lightning fighter aircraft.

[39] On 20 April 1943, Tonne was killed in a flying accident, this event had a significant effect on Müller and influenced his health.

Gruppe, and the pilots and ground crew were transported by sea to Sicily shortly before the German surrender in North Africa on 12 May.

Gruppe and ordered to relocate from Italy to the Venlo airfield, officially taking over command on 26 February.

He now fought in the aerial battles in Defense of the Reich, just after the USAAF offensive dubbed "Big Week" came to an end.

Gruppe was transferred to Salzwedel where it was augmented by Sturmstaffel 1, an experimental unit flying the so-called Sturmböcke (Battering Ram) up-gunned Focke-Wulf Fw 190 A-7 and A-8 aircraft.

In total, the Eighth Air Force mustered 730 four-engined bombers and 801 fighter aircraft in the first, full-scale daylight attack on Berlin.

Gruppe, led by Müller, was one of the Luftwaffe units dispatched to counter this attack and claimed the destruction of seven B-17 bombers, four B-17 Herausschüsse (separation shots–a severely damaged heavy bomber forced to separate from its combat box and which the Luftwaffe counted as an aerial victory) and one North American P-51 Mustang fighter for the loss of one Bf 109 G-6 damaged.

Gruppe against an attack of the Eighth Air Force targeting the Vereinigte Kugellagerfabriken AG ball bearing factories in Erkner, south of Berlin.

In defense of this attack, JG 3 lost 16 pilots killed in action including the Geschwaderkommodore (wing commander) Oberst Wolf-Dietrich Wilcke.

[54] Müller, who had his regular wingman Leutnant Dieter Zink transferred to the Geschwaderstab (headquarters unit), claimed eighteen further victories, seventeen of which were four-engined bombers.