On 3 October 1940—two weeks after his wedding—Crombie embarked on a ship in Sydney, bound for Canada. He arrived three weeks later, where he completed an additional two months of advanced flight training. Crombie was commissioned as a pilot officer on 17 January 1941 and set sail for the United Kingdom nine days later. In May, he was attached to the Royal Air Force's No. 25 Squadron, piloting Bristol Beaufighters. While with the unit, he flew his first operational sortie on 13 June; he was promoted to flying officer the next month. Crombie continued to serve with the squadron until October, by which time he had flown a total of twelve operational sorties over the European theatre, including two raids; one over Germany, and the other to Brest, France.
