When the Netherlands surrendered to Germany in World War II, the Luftwaffe started using the field, significantly expanding and upgrading the base.
The Focke-Wulf Fw 190, Messerschmitt Bf 110 and Junkers Ju 88 were operated from the airfield, and anti-aircraft guns were installed.
A railway from the nearby town of Wolfheze was constructed, initially to facilitate the transport of building materials for a Luftwaffe command bunker and later to supply the base with munitions.
In 1944 the airfield was extensively bombed by Allied forces in preparation for the Battle of Arnhem, and flying activity from the base was suspended by the Germans.
The increased importance of Deelen resulted in it once again receiving the designation of Air Base (Dutch: Vliegbasis).