The airmen use the term UFO when the term UFO wasn't invented until 1952."Use of "UFO" instead of "flying saucer" was first suggested in 1952 by Capt. Edward J. Ruppelt, the first director of Project Blue Book, who felt that "flying saucer" did not reflect the diversity of the sightings.
The B-17, unlike the later B-29, was not pressurized. The B-17 pilot, early in the film, says they are flying to 20,000 feet - and yet the crew is not wearing oxygen masks. These are required if you fly much above 12,000 feet.
The downed airmen make radio contact with an Allied airbase, who identify themselves as being "8th Airborne". There was no 8th Airborne; the writers likely meant 8th Army Air Force.
The actions the Flight Engineer is ordered to take in response to the engine fire in the B-17 would have been performed by the pilot or co-pilot.
The Army Air Corps flight crews are shown in basic flight suits; not cold weather suits. Equipped as shown they would have frozen to death within a few minutes at cruising altitude.
The interior shots of the B-17 show far, far too much space. The interior appears to be 25 to 40 percent over sized.
The Army Air Corps enlisted personnel wear no rank insignia.
(at around 1h 15 mins) In the scene on the bridge with the mortar, as the German fighter strafes the soldiers, you can see that its propeller is not moving.
The German accents are brutally unconvincing, there was clearly no training or coaching involved.
High tension power lines seen several times during the bridge scenes.
US flags have 50 stars in the movie; however, the US had 48 states (and thus a 48-star flag) during WWII. Alaska and Hawaii did not become states until 1959.
One of the characters is listed as a B-51 Pilot. The B-51 was a low flying fast attack jet bomber that had not been invented yet (as opposed to the high flying famous B-52) The production probably meant "P-51" pilot, which was the famous "Mustang".
When the B-17 goes down, the radioman uses the plane's nickname instead of its call sign when sending out a mayday. Further, the mayday is sent over a voice radio which was used for short range communications between aircraft. A mayday would be sent using Morse Code with a long range radio.
In the church when the exposition about the gargoyles occurs, the girl states that the small ones are "flesh and blood"; yet, moments before one was hit by a shotgun, and shattered as if made of stone.