From this web page, http://www.nationmaster.com/encyclopedia/Typewriter we find the following history of the electric typewriter:
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Selectric mechanisms were widely incorporated into computer terminals in the 1970s, ... .
Later models of Selectrics ... introduced selectable "pitch" so that the typewriter could be switched among pica ("10 pitch"), elite ("12 pitch"), and sometimes agate ("15 pitch"), even in one document. Even so, all Selectrics were monospaced -- each and every character was the same width. Although IBM had produced a successful typebar-based machine, the IBM Executive, with proportional spacing, no proportionally-spaced Selectric office typewriter was ever introduced. There was, however, a much more expensive proportionally-spaced machine called the Selectric Composer which was considered a typesetting machine rather than a typewriter.