When getting the directing job, Enzo and a writing partner took a week to rewrite the overlong script to make it more action orientated and less of a "soap opera."
Alberto De Martino was originally going to direct the film and Enzo G. Castellari was originally just hired to direct the special effects sequences inter-cut with stock footage of the actual battle of Britain using the split screen method. Up viewing the split screen sequences the producers were so impressed that they hired Enzo to direct the entire picture.
In the opening scene, the British are shown using a bazooka-like weapon to knock out the tank column. In 1940, there was no such thing as the weapon seen here.