Not sure which came first, the chicken or the egg. The 707 or the C-135 family.
http://www.aerospaceweb.org/aircraft/transport-m/c135/
The C-135. Boeing knew the flight characteristics of the B-47's and B-52's they built for the USAF and knew that the existing KC-97's would not be adequate to refuel the strategic bombing fleet. The KC-135 fuselage diameter was set after the air force requested a larger fuselage than the Dash-80 demonstrator that had been used to test docking without any fuel transfers. Boeing had intended to offer the same plane to the civilian market using all the same tooling, but Douglas got orders for its DC-8's by making its fuselage a few inches wider and able to handle 6-abreast seating. Boeing made the decision to build a new fuselage cross-section just one inch wider than the DC-8's for the civilian 707.