The Cliff's Notes version is, Danelectro and Fender began offering six-string basses, also known as baritone guitars, in the early '60s. As their name suggests they were guitars with heavier strings tuned down an octave. In '68 or so, Hagstrom began offering an 8-string bass that mimiced the lowest two-thirds of a 12-string guitar but tuned down two octaves. Somewhere around that time, after their sale to CBS in 1964, Fender began offering a five-string bass with a high-C string.
This idea was seized upon by our good friends at Alembic and tweaked by session bassist Jimmy Johnson, who ordered an Alembic five-string with a low B in about 1974...Mica or Susan would know more exactly than I.
Independently, at the same time, bassists Anthony Jackson and Stanley Clarke began working with New York luthier Carl Thompson to develop, respectively, the modern six-string bass (tuned B-E-A-D-G-C) and the piccolo bass (tuned E-A-D-G one octave above a standard bass).
That's it in a nutshell--this history can be found in The Bass Book, a coffee table book of history and bass porn that may still be available for you to buy and cite for your paper. If you can't find a copy and happen to live in the US, e-mail me and I'll let you borrow mine if you promise to return it.