The Go programming language was designed inside Google. The Go FAQ records the exact starting point: “Robert Griesemer, Rob Pike and Ken Thompson started sketching the goals for a new language on the white board on September 21, 2007.” What began as a part-time exploration grew into a full-time project over the following year.
The FAQ traces the early timeline in detail: by January 2008 Ken Thompson had started on a compiler, Ian Taylor independently began a GCC front end for Go in May 2008, and Russ Cox joined later that year to help move the language from prototype to a working reality. Go then became a public open source project on November 10, 2009.
The reason it was built at Google is documented in Rob Pike’s 2012 article “Go at Google,” which states that “the Go programming language was conceived in late 2007 as an answer to some of the problems we were seeing developing software infrastructure at Google” — including slow builds, tangled dependencies, and concurrency needs that the designers felt C++ and Java did not serve well at the language level.