The Space Shuttle was originally called 'station shuttle', as it was meant to service a large orbital space station (planned to be launched in the 1970s). Unfortunately for NASA, its budget was cut during and after the Apollo missions, and it decided to postpone the station, but keep the shuttle. The MIT course on the Space Shuttle is very informative; you might find the episode on the decision to build the shuttle particularly interesting.[1]
I don't know of any shuttle study that envisioned it being SSTO. The original shuttle configurations did contain a liquid-powered 'flyback booster', which was a first stage with the ability to fly back to the launch site. Many of these configurations also had retractable jet engines mounted on the shuttle orbiter, as well as fuel and/or oxidizer tanks in the orbiter, but all these options were nixed because of payload, cost, and schedule requirements. The quick turn-around was originally viewed as desirable, but other things were made higher priorities, and they gradually moved away from that objective.
[1] https://www.youtube.com/watch?v=bOAyzURugaw&list=PL35721A60B...