Actually they are recce modified KC-135s. That’s how they started life, before they were converted. Many retained the boom operators station, although the boom had been removed.
The -135 and 707 are different airfames with a common ancestor, the Boeing Dash 80. The 707 was, in USAF service, used as the basis for the C-137 (including those used in the Air Force One role), the E-3 Sentry and E-8 JSTARS. It’s also used as the basis for the Navy E-6 Mercury airborne command and control aircraft.
All of the converted KC-135’s either crashed or are retired.