Yes it was. t was filmed at Bovingdon, near Hemel Hempstead, Hertfordshire, UK, the same airfield (now a prison) used for a number of movies, including '633 Squadron' and 'Hanover Street'.
There's a great 'still', taken from the top of the Control Tower, showing the 'Fort' just a couple of feet off the deck, in Roger Freeman's 'B-17 at War' book.