Factual error: The Zeppelin shot down in this movie is labeled L32. The real L32 was shot down over England (near Billericay) , rather than France, and during night raid, not in daylight.
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.
Continuity mistake: Many of the flying scenes have unrealistic changes of altitude. The planes will be able to see people on the ground, and in the next action shot, they will be thousands of feet in the air.
Factual error: One of the characters is seen boarding the ocean liner Aquitania in New York. This ship was not in passenger service during World War One. And the Statue of Liberty is visible behind the docked ship. The pier used by Cunard Line in NYC would not have offered this view.
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. (01:16:00)