There have been people working on spaceplanes continuously since the Nixon administration. Shuttle was a spaceplane but there were/are lots of other programs that fit that title.
Personally, I put the beginning of this program with the NASA Lifting body program[1]. The most famous one for me being the M2-F2 which was shown crashing in the opening sequence of the six million dollar man TV series.
Those who played the classic computer game Buzz Aldrin's Race into Space (which was completely open-sourced a few years ago: see https://www.raceintospace.org/) will remember that one of the key decisions in that game, which was all about building a space program that could get you to the moon, was whether to use traditional capsules (Gemini/Apollo on the US side, Voskhod/Soyuz for the USSR) in the mid-to-late game, or to go with reusable lifting body-type vehicles instead. The American player could build the "XMS-2", a fictional vehicle derived from the X-20 Dyna-Soar (https://en.wikipedia.org/wiki/Boeing_X-20_Dyna-Soar), and the Soviet player could build a similarly fictional vehicle based on the MiG-105 (https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-105).
It was a big decision gameplay-wise, as capsules were much cheaper to research and purchase, but the reusability of mini-shuttles meant you didn't have to buy a whole new crew vehicle for each shot. So if you could scrounge up the money to buy a few in the mid-game, you could free up big bucks in the late game that would otherwise have gone towards buying all those capsules.
Maybe officially, but I definitely know people who were working on a Space Plane program in the early 90s.