Overlord's first sequence, which sees the soldiers jumping from a burning plane, was done by rigging a plane on a gimbal, actually blowing up the front, tilting it as if it were actually falling through the air, and sending stuntmen tumbling through real fire.
The movie featured more practical effects rather than the standard cgi effects most movies use. This was done to get a better reaction from the actors involved in the scenes where something gruesome would happen.
Operation Overlord was the code-name for the Allied operation for the Battle of Normandy, which launched the successful invasion of Nazi-occupied Europe. This, alongside Operation Neptune, would become known as D-Day.
The plaque on the wall of the lab says "Al Pascal Moreau," which is an homage to famous scientists connected with the plot. "Al" most likely refers to Albert Einstein, the physicist who encouraged the US to develop the atom bomb during WWII. Blaise Pascal was a 17th-century French mathematician and scientist who invented the syringe. And Moreau is a fictional scientist from H.G. Wells' novel "The Island of Doctor Moreau," who uses genetic mutation to create horrifying hybrid creatures.
Original version of the script written in 2013 by screenwriter Billy Ray was very different in some parts than the final movie, before it was changed over the years and after another writer Mark L. Smith was brought in to polish the script. One of the biggest differences included only having Boyce, Ford and Chloe as main heroes, and far more focus on horror elements of the story. Entire third act and action scenes in the ending were almost completely different than the ones in the film. For example, Boyce, Ford and Chloe were fighting against even more undead Nazis inside the church and laboratory and using every weapon they had or found trying to stop them, at one point Boyce even throws some grenades at Nazis but when that doesn't work he has to use giant sword to decapitate them, and then uses the flamethrower to burn last ones before blowing them up for good.