Microsoft insiders have been calling the project "Threshold" while publicly it's been called "Windows 9". Either way, "Whatitsname OS" will get a full release early next year.
The new name is part of redirecting Microsoft's image away from software and PCs.
Under new boss Satya Nadella, Redmond is placing an emphasis on selling services across all devices, particularly Microsoft's mobile and cloud services.
Observers say Nadella may have a difficult task promoting the new system. With less than 14pc of PC users, its most recent foregoers Windows 8 and 8.1 are a distant third in the OS race, even though new Windows 8.1 devices are said to be as cheap as mainstream computers have ever been. Windows XP, which isn't supported any more, still has 70 percent more users.