Factual error: The film is set up until the USA enter the war, which was April 1917. The Germans always appear flying Fokker Dr.I triplanes, which flew for the first time in August and only appeared in any significant numbers over the front in November. Granted, the triplanes were a great way of letting the audience know who were the good and bad guys when so many similar-looking aircraft were together in the sky at the same time and in such haste.
Factual error: All the Fokker Dr. 1's in this movie (save for one) are red. This was most likely done because the best known Dr. 1, the one flown by The Red Baron, was red. It was however, a pretty rare color to see on German planes. There might have been a few copycats during the war, but there were no entirely red squadrons.
Factual error: While Blaine Rawlings (James Franco) is speaking with the Brothel owner, the song "How ya gonna keep 'em down on the farm" is playing in the background. The movie takes place in 1916. However, the sheet music for that song was not published until 1918. The Arthur Fields recording was made on February 2, 1919.
Other mistake: In one of the air battles, the one where our heroes come to aid to a platoon of infantry harassed by German Fokker D.I's, the plane of Lowry is under fire after he downed an enemy airplane. In one shot, you can see he gets multiple hits just behind the pilot seat. Since these planes are barely armoured, say not at all, Lowry should have been punctured by the bullets at his back as well, instead of being completely fine.