In September 1941, Dundas was posted as an instructor to No. 59 Operational Training Unit, but his scruffiness and unruly pet dog did not endear him to the commanding officer, Group Captain Stanley Vincent, and he was transferred promptly as a flight commander with No. 610 Squadron. Promoted to flight lieutenant (war substantive) on 2 October, he was subsequently promoted to acting squadron leader and posted as commanding officer of No. 56 Squadron RAF in December, the first to be equipped with the Hawker Typhoon. He was mentioned in despatches on 1 January 1943, and was promoted to squadron leader (war substantive) on 11 February. Posted to the Mediterranean in 1943, he led No. 324 (Spitfire) Wing from Malta and through Italy. He was awarded his first Distinguished Service Order (DSO) on 3 March 1944. In June, Dundas was promoted to the acting rank of group captain at the age of only 23. He led No. 244 Wing from June 1944, and was promoted to wing commander (war substantive) on 11 May 1945. His war time score was 4 enemy aircraft destroyed, 6 shared destroyed, 2 shared probables, and 2 and 1 shared damaged.