Microsoft grew out of a programming language for the first popular personal computer. Microsoft’s own “History of Microsoft” timeline records that on January 1, 1975, “the MITS Altair 8800 appears on the cover of Popular Electronics,” and that “the article inspires Paul Allen and Bill Gates to develop a BASIC language for the Altair.” By February 1, 1975, the two had “completed Altair BASIC and sold it to Microsoft’s first customer, MITS of Albuquerque, New Mexico.”
The company’s name came slightly later. Microsoft’s timeline notes that on July 22, 1975, Gates and Allen “sign a licensing agreement with MITS regarding the Basic Interpreter,” at a point when “the name Microsoft has not yet been chosen.” The earliest written use of the name is dated July 29, 1975: “In a letter to Paul Allen, Bill Gates uses the name ‘Micro-soft’ to refer to their partnership. This is the earliest known written reference.”
The turn from language vendor to operating-system giant came with the IBM PC. IBM’s history of that machine records that for its 1981 personal computer, “Microsoft provided the OS, which would later become known as MS-DOS.” MS-DOS, and later the Windows family, carried Microsoft to dominance over the personal-computer software market in the decades that followed.
Note on dates: this entry uses a year-level date because the verified primary source documents the 1975 founding events month by month (January through July 1975) rather than asserting a single founding day.