the man who never was (1956)

"The major. May he have a safe, but successful journey."
An British intelligence officer (Clifton Webb) comes up with a cunning plan to fool the Germans as to where the Allies were intending to begin their land invasion in 1943. The plan was simple; get a body with bogus plans into the hands of the Germans. But how to do it, so that the body and its life story will convince the enemy agents scrutiny.
Based on intregingly real events, this is a classy, though uninvolving version of the story.