I don't really want to split hairs but the USAF RC-135 is NOT a 707 derivative.
More accurately the U.S. Air Force's
C-135 Stratolifter is not a 707 variant, but was developed in parallel to the 707 from the original Boeing 367-80. As such there are a lot a similarities, and lots of differences. Specifically it has a narrower fuselage and was fitted with different engines from the get go (J57s originally, with virtually all now running CFM-56s).
More importantly perhaps is the fact they have the loving maintenance care of the Air Force, which tends to get rather upset if any of their planes break or fall apart. (Not that it doesn't happen, but all sorts of questions tend to get asked when they do...).
- GY
