At the time this was one of the highest budgeted films ever to be made in Italy (behind The Good, the Bad and the Ugly (1966) which was partially financed with American money through United Artists).
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. Upon viewing the split screen sequences, the producers were so impressed that they hired Enzo to direct the entire picture.
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."
Enzo G. Castellari original did not know what a "split screen" method was when he was asked to direct the effects sequences utilizing stock footage using a split screen method. He was shown the American films The Thomas Crown Affair (1968) and The Boston Strangler (1968) which both utilized split screen.