Electro-Spanish Model B
The 1935 Electro-Spanish (Bakelite) Model B was, commercially, the most popular electrical stringed instruments manufactured by Rickenbacker. Though not entirely solid - it had thick plastic walls and a detachable Spanish neck. The instrument was created to eliminate the feedback found in conventional electrification of stringed instruments.
The "Electro-Spanish Model B" set the stage for solid body guitars to develop; including the notable "Les Paul" whose electric solid-body would reach mainstream notoriety in 1952, produced by Gibson Guitar Corporation.
References
Electro-Spanish Model B Wikipedia(Text) CC BY-SA