Düllberg was born on 28 March 1913 in Unna, at the time in the Province of Westphalia within the German Empire.

[4] On 1 August, Düllberg shot down the Bristol Blenheim bomber R2774 from the Royal Air Force (RAF) No.

Gruppe (2nd group) of JG 27 was ordered to an airfield at Crépon, France for combat in the Battle of Britain.

He replaced Hauptmann Albrecht von Ankum-Frank who had been killed in a crash landing on 2 August.

[7] On 18 August, also known as the Hardest Day, Düllberg led the Gruppe on a Junkers Ju 87 dive bomber escort mission to Thorney Island.

Near the Isle of Wight, the flight was intercepted by two squadrons of Hawker Hurricane fighters, shortly after Supermarine Spitfire engaged too.

During this encounter, I. Gruppe of Sturzkampfgeschwader 77 (StG 77—77th Dive Bomber Wing) suffered heavy losses while Düllberg claimed one of the Hurricanes shot down.

Gruppe arrived at Döberitz, located approximately 10 kilometers (6.2 miles) west of Staaken, on 24 July.

Staffel under command of Düllberg began the relocation, arriving at Ayn al-Ġazāla on 1 October.

[11] On 22 October, Düllberg shot down the Curtiss P-40 Tomahawk AN230 from the South African Air Force (SAAF) 2 Squadron.

[14] On 22 November 1941, Düllberg was wounded in aerial combat south of Sidi Rezegh resulting in a forced landing in his Bf 109 F-4 trop (Werknummer 8466—factory number).

Although severely injured, Düllberg flew his damaged aircraft back to the airfield at Ayn al-Ġazāla.

In mid-October, JG 76 was transferred from Freiburg to the southeastern sector of the Eastern Front and placed under the command of Luftflotte 4 (Air Fleet 4).

[4] According to Obermaier, Düllberg was credited with 45 aerial victories claimed in approximately 650 combat missions.