Boeing has been building jetliners for 60 years without windows (except for the flight deck and exit portals) - KC-135, 747F, 757F, 767F, 777F
I understand that, but Boeing doesnt build a different structural frame for the F models do they?
Now going back, I did say saving 25% on the fuselage structure? Thinner elements, no bracing and no windows? Not to mention ease of construction...