Whilst upright bass strings were originally made of gut, the development of the first mass-produced electric basses in the 1950s by Leo Fender and George Fullerton meant that cords made of ferrous metal would be required to interact with the pickups. Early prototypes of the Precision Bass used gut with an iron wire wrap just above the pickup magnets.
When the final product shipped in 1951, full-steel electric bass guitar strings became widely available for the first time. It coincided with a shift by upright bassists away from wound catgut towards steel. This, along with the similarity to electric guitar, led to a boom of players transitioning to the more portable, 'horizontal' bass.