While the vast majority of Boeing Model 717s (C-135s) bought by the U.S. Air Force were tankers, there were also a much smaller number of C-135s built as transports -- the first USAF jet transports. 15 C-135As served and then 30 improved C-135Bs with turbofan TF33 engines were purchased. These provided an interim long-range jet transport capability for several years until the Lockheed C-141 was introduced.
The C-135 Stratolifters were not ideal transports; for the transport of cargo, the load floor was very high and it was difficult to get cargo in and out. (See the photo of the cargo door above in the KC-135 post). Once the C-141 was introduced, the transport C-135s were used elsewhere. Some were converted into command posts/VIP aircraft or testbed aircraft, while others became the reconnaissance variants still used today. Note that even the VIP transports had few windows, reflecting the origin as a tanker.
https://en.wikipedia.org/wiki/C-135_Stratolifter