Windows Phone 8 codenamed ‘Apollo’ has been in development since 2011. The previous major branch of Windows Phone codenamed ‘Mango’ RTM’d in July 26 th , just over one year ago. Bringing along a new core architecture built upon the NT kernel, the Windows Phone 8 OS will allow Microsoft to adopt cutting edge technology and heavily compete with Android and iOS in terms of features and hardware flexibility.

We're also told that this RTM date was planned and that Windows Phone 8 is "on time".

Tonight we can confirm via two known sources that Windows Phone 8 has RTM’d. For those of you unfamiliar with the term, RTM stands for Release to Manufacturing (aka “going gold") and is one of the final steps in getting the software to market.

Windows Phone 8 devices: (L to R) Lumia 920, Lumia 820, HTC 8X, ATIV S

While the full feature list of Windows Phone 8 has not yet been revealed, users were treated to a sneak preview back in June at the Windows Phone Summit. Some of the features introduced include multi-core CPU support, near-field communications (NFC), 720p and higher resolutions and native C++ development.

With Windows Phone 8 now gold, all eyes are now on the software development kit (SDK) for developers. Although some select users were able to download it this week, the SDK has yet to be finalized. We’ve heard that newer builds are now going out to those with access and with the OS now finalized, the SDK can presumably get polished off for general release.

Team members sign off

The next step with the OS being delivered to OEMs will be for them to iron out any kinks and optimize drivers for their hardware. There still may be some back-and-forth between the OEMs and Microsoft as bugs and other minor issues are discovered but for all intents and purposes, the OS is now locked and feature complete.

What all this means for finalized hardware for carriers is exactly what we’ve been expecting: a November release for many carriers, following the “big unveiling” of Windows Phone 8 possibly at BUILD ’12 in Redmond. Whether those dates are November 2nd or later remain to be seen, as carriers still need to approve the devices for general sale.

Updated: 'Signing off' images from 'coolrick'; via LiveSino.net; Thanks, Picturepan2.

Windows Phone Central of course will be live at BUILD 12 with full coverage of all events.