Not an aviation film, but one of my favorites is the 1936 movie, "The Road to Glory", starring Fredric March, Warner Baxter and Lionel Barrymore. It's a very realistic depiction of the French Army and trench warfare, directed by Howard Hawks and co-written by William Faulkner (pretty good credentials); supposedly based on a previous French movie. Not available on DVD or VHS, but if you're lucky you might catch it on TCM; TNT used to show it occasionally. It has a very nasty attack across No-Man's Land, and a good look at life in the trenches. It even has a scene of French Poilus nervously listening to Germans tunneling underneath their dugout to plant a mine - the only depiction of this WWI tactic I've ever seen in a movie.
In many ways it follows some of the same themes as "Paths of Glory" and "A Very long Engagement".