Rennie joined the Royal Air Force in 1941, training as a fighter pilot in the US under the Arnold Plan. While at Napier Field in Dothan, AL, for his advanced flight training he was asked by a fellow trainee, Scotsman Jack Morton, what he did in civilian life. Rennie told Morton and the other pilots gathered around that he was a movie actor. They stared at him in disbelief, then broke out in a chorus of laughter. A couple of nights later Rennie and his classmates went into town to watch a movie,
Ships with Wings (1941). Not long into the movie, and much to the surprise of those seated with him, Rennie appeared on the screen as Royal Navy pilot Lt. Maxwell.
