The computing giant let the launch date slip at the Japan Developer Days for Windows 8 conference, and later offered clarification via Twitter.
Microsoft's next OS will come in three different versions, two for Intel-powered computers and one for devices running ARM processors.
Windows 8 and Windows 8 Pro will replace Windows 7's system of Home Basic, Premium, Pro and Ultimate versions, while Windows 8 RT will be exclusive to ARM devices.
Windows 8 is geared towards the consumer market, while Pro will offer encryption, virtualisation, PC management, domain connectivity and the Windows Media Centre add-on.
Windows 8 RT edition will primarily be aimed at tablet users, offering touch-optimised editions of Microsoft Office programmes Word, Excel and Powerpoint.
Each version of the OS will be backed by the Windows Store, enabling users to purchase applications for desktops, laptops and tablet computers.
Windows 8 Preview will land in the first week of June.
Watch an introductory video to Windows 8 below: