Microsoft released Altair BASIC in 1975 as its first major software product.
Paul Allen and Bill Gates wrote the interpreter for the MITS Altair 8800, an early commercially available personal computer. Gates demonstrated the software to MITS, which agreed to distribute it. The product helped establish Microsoft as a software company rather than a hardware manufacturer.
Altair BASIC was designed for the Altair's Intel 8080 processor and was delivered on paper tape before later versions used other media. The name Microsoft was originally written with a hyphen, “Micro-Soft,” in the company's early history.
A common mix-up is assuming Windows was Microsoft's first product. Windows did not appear until 1985, a decade after Altair BASIC. The company's early success came from licensing software to computer makers, a strategy that later shaped the personal-computer industry.