Joe Belfiore, Corporate Vice President, Operating Systems Group at Microsoft said that the phone builds have not been as far along as PC builds.

Build 2015: Microsoft opens Windows 10 operating system to Apple, Android apps

Belfiore also said that from the device view, their main focus is to kick off the Windows 10 launch wave with a great launch on the PC. Users should expect that the other devices like phone, HoloLens, Xbox, Surface Hub will be staggered.

Windows 10 is continuing the same trend of Microsoft's PC release of Windows 8.1where PC release was earlier than the phone release.

In addition, Microsoft said that some features won't be available on Windows 10 at launch.

Microsoft reveals 'Edge' brower to replace its Internet Explorer

It is not clear how many features will miss the initial launch window, but Microsoft revealed that launch features start in the summer and increase in the fall. Also, Microsoft's new Edge browser extension support will arrive after the summer.

Microsoft still has not revealed exactly when the company will launch Windows 10.