One of those milestones is to open up the Software Development Kit ( SDK ) to developers for some initial feedback. Windows Phone Central has learned that Microsoft is now inviting a limited number of developers, under NDA, to begin early testing of the new 8.1 operating system and its SDK.

Windows Phone 8.1, the next milestone for Microsoft’s mobile OS, is rumored to be announced April 2 in San Francisco, California . That’s only 7 weeks away, putting Microsoft on a tight schedule to line up all of their eggs for the big reveal.

SDKs are the tools used by developers to design apps for the OS, leveraging new capabilities, including screens resolutions, APIs, or new features like the previously leaked Action Center and ‘intelligent’ notifications. Getting feedback from developers on those tools and any errors helps mitigate any initial launch bugs that may have been overlooked by ‘dogfooding’ the SDK internally at Microsoft.

Developers who agree to the NDA are of course forbidden to talk about the developer program, including even its existence or whether they are a part of it. Still, we imagine that as Windows Phone 8.1 gets into more hands, more details of Microsoft’s forthcoming OS will inevitably make its way on to the web, similar to this weekend’s leak of the Action Center.

In addition to the confidential SDK Preview Program, Windows Phone Central can now report that the public Developer Preview Program is slated for an April 2nd announcement at Build. That program, an extension of the current Update 3 preview, will allow regular users to download the early bits of Windows Phone 8.1 on that day (or that week) to their current Windows Phone, bypassing the carrier approval process, which could delay the 8.1 rollout by a few months. While users would get the latest OS, they will however be missing any optimized firmware, which would be updated at a later time.

In response to a media inquiry, Microsoft had the following to say regarding the SDK invites:

“We regularly involve our developer community in a variety of private programs. We have nothing to share broadly about our recent developer outreach.”

Finally, Microsoft seems to be ramping up plans for Windows Phone 8.1 in general, including holding an NDA meeting with partners at the end of February for those who can't attend Mobile World Congress. That meeting will feature a roadmap session and technical briefing on Windows Phone, prepping partners before the expected public announcement in April.