During the funeral is mentioned that Matrac's son is 5 years old. Assuming that was born in 1940 (his father condemned in 1939 plus nine months of pregnancy) the movie depicts 1944 or 1945. By that time France have been already freed.
We see the plane during bombing runs being attacked by anti-aircraft artillery and fighters at the same time. This would not happen as the fighters would stay outside the area when AAA is firing on the planes; as they would be shot down by their own guns.
In the film, the aircraft used by the Free French squadrons are American-built Boeing B-17 Flying Fortresses, and are described as such by Capt. Freycinet. There actually were two Free French heavy bomber squadrons based in England: Groupes de Bombardment GB II/23 "Guyenne" and GB I/25 "Tunisie," which were part of the Royal Air Force as 346 and 347 Squadrons respectively. However, they did not fly Fortresses. Their aircraft was the British-built Handley Page Halifax. Obviously, for Hollywood purposes, Fortresses were easier to obtain than Halifaxes.
The German aircraft attacking the freighter is correct for the period with one exception. The interior views of the air gunners, they are using the wrong weapons. A 20mm mg on a swivel mount is in the forward part of the gondola. In the rear part of the gondola, a 13mm mg is mounted. The weapons shown being used are U.S. 50 cal mg.
When describing French Guiana to another escapee, the construction by inmates of Route Zero (the road to nowhere) in the jungle is shown with an escape attempt, where the escaping inmate is shot by a guard and left in the swamp. Immediately, an broad-snouted alligator is shown swimming toward the hapless victim who was just shot. However, there are no alligators in French Guiana, or anywhere else in South America for that matter. Alligators only exist in the southeast mainland coastal region of the USA and along the Yangtze River in China, and are easily distinguishable from crocodiles by their broad snout (whereas crocodiles have long narrow tapered snouts). The species more likely to be found in French Guiana was the caiman, a smaller relative of the American Crocodile.
Speaking of the firing of the guns from the plane, also, note that no ammunition moves through the gun as well. When the men are being questioned in the boat, the boat is shown swaying back and forth as if at sea but the lamp in the cabin doesn't move at all. Ship lamps on all sailing vessels are made to swing to remain level while the boat rocks back and forth, so the lamps should be moving in the opposite direction of the ship.
When Mattrac is pointing his fifty cal. machine gun out the waist gun window, we hear machine gun sounds, but the gun is not firing. The 50 cal. M2 machine gun produces a great deal of vibration when it is being fired. The gun did not vibrate one bit.