You might find this surprising, but most of creating a shot that could pass for France 1940, is in the process of the shot, not in post.
Find your subject, in this case a Bf109E of the Flying Heritage Collection in Everett, Washington. Find an angle that allows inclusion of any details that help sell the illusion, notice the 88mm AAA piece in the background. Then frame such that no elements that ruin the illusion are included. Something like the building is allowable, although I would rather it not be there, there is nothing about it that screams “2011.”
Finally, process it in Silver Efex Pro 2. I use one of the lens vignettes and a sepia tone for my “period” images, always desaturating the “paper” tone. I feel the default makes the whites too orange. Then clone out anything you weren’t able to frame out that ruins the illusion, unless your intended use would make such removal unethical. I only cloned out sensor dust in this image, if you look really closely there is a piece of avionics gear that is a candidate for cloning.