The narrator (Eugene Kern) claims that Britain was last conquered more than a thousand years ago. The latest conquest of Britain was in 1066, 878 years before the film was released.
As the Forts return to their base, one is shown with its rudder and a good chunk of the fin blown away. In the closeups on the ground, what's left of the tail number reads backward, showing that the film for those scenes was flipped right/left. That plane is 41-9020, which was reassigned as a target tug 32 days before Memphis Belle flew her 25th mission.