One way to ensure that there will never again be a five year gap between Windows releases, as with XP and Vista, is for Microsoft to start work on a new iteration of its platform even before the latest version is finalized.

This happened with Windows 8, which the software giant confirmed that it started planning for as early as the summer of 2009.

For those not exactly keeping track, Windows 7 launched in late 2009. Vista’s successor was released to manufacturing in mid-2009, so even as it was putting the finishing touches on Windows 7, Microsoft was already looking forward to Windows 8.
Windows 8 Planning Started in mid-2009 - Softpedia