Which American billionaire co-founded Microsoft with Bill Gates?

Paul Allen co-founded Microsoft with Bill Gates in 1975.

Allen and Gates had been friends and computer enthusiasts since attending Seattle’s Lakeside School. After seeing the Altair 8800 featured in the January 1975 issue of Popular Electronics, they recognized an opportunity to develop a BASIC programming language for the new microcomputer. Their demonstration worked, leading MITS to distribute Altair BASIC.

Microsoft was established on April 4, 1975, in Albuquerque, New Mexico. Allen suggested the early name “Micro-Soft,” combining “microcomputer” and “software.” Gates became the company’s chief executive, while Allen played a major role in its early programming and business decisions.

Allen left Microsoft in the early 1980s after being diagnosed with Hodgkin lymphoma, but his retained shares made him extraordinarily wealthy when Microsoft went public. Steve Ballmer later became a major Microsoft executive, but he was not a co-founder.