While visiting Stratasys' HQ in Minneapolis, we were able to meet CEO and Founder Scott Crump, who told the story of how he invented the Fused Deposition Modeling (FDM) process many years ago.
He was inspired by two things: the need to be able to make prototypes inexpensively and two-dimensional plotters. He had the idea that if a 2D plotter could be persuaded to create incremental layers, one would be able to gradually create arbitrary objects.
He began by trying to simulate the process on his kitchen table using a handheld hot glue gun on wax paper. Once the fundamentals of the process were understood, he modified a 2D plotter to perform extrusion of wax.
The approach worked, but software had to be developed to drive the machine. AutoCAD version 2.3 was the first design software used by Crump for designing models. After seven prototypes, a reliable unit was produced and shipped in 1991. In 1992 Stratasys shipped four units, and it's grown exponentially since then.
Of course, the resulting wax models weren't particularly strong and Crump switched from wax to ABS thermoplastic in 1993, which is still used today in Stratasys' machines.