Who was the first jazz group to mix jazz and rock?

Miles Davis is generally considered the forerunner of fusion jazz, which combined jazz instrumentation with rock's electric instruments. His "electric era" began in 1968, and his first fusion album, "In a Silent Way," was released in 1969.

A British group called Soft Machine also began developing fusion jazz around the same period. Their second album, "Volume II," features lengthy jazz improvisations played loudly on electric instruments. Live recordings of the group playing these pieces live before the release of their second album arguably makes them the first jazz fusion group.