Given the below context:  A few months before the D-Day landings during the Second World War, the British government decides to launch a campaign of disinformation; spreading a rumour that the landings just might take place at a location other than Normandy. The details of the operation (actually, there were several such operations) are handed to two intelligence officers, Colonel Logan and Major Harvey. They are initially unable to devise such a plan – but one night, Harvey sees an actor at a London theatre, putting on a convincing impression of General Bernard Montgomery. Logan and Harvey discover that the actor is M. E. Clifton James (who plays himself in the film), a lieutenant stationed in Leicester with the Royal Army Pay Corps and that he was a professional actor in peacetime. He is called to London, on the pretext that he is to make a test for an army film, and a plan is devised that he should tour North Africa, impersonating 'Monty'. 'Jimmy' as Harvey calls him, is doubtful that he can carry off an impersonation of Montgomery, especially with his air of command, but with time running short and no options open to him, he agrees. Disguised as a corporal, he spends some days at Montgomery's headquarters and learns to copy the general's mannerisms and style. After an interview with the general himself, he is sent off to tour North Africa. Accompanied by Harvey, who has been 'promoted' to brigadier for his cover as Montgomery's aide-de-camp, 'Jimmy' arrives at Gibraltar, where the governor, who has known the general for years, can't get over the likeness. To further foster the deception, a local businessman and known German agent, Karl Nielson, is invited to dinner, knowing that he will spread the information. This happens quickly and their aeroplane is (unsuccessfully) attacked on leaving Gibraltar.  Guess a valid title for it!
I Was Monty's Double (film)