The Windows Phone development team officially signed off on the release to manufacturing (RTM) build of “Mango” – the newest version of the Windows Phone OS. By this fall, Microsoft’s mobile OS will available on handsets worldwide.

The RTM stage marks the end of development for Windows Phone Mango which is now undergoing carrier testing. Now Mango is in the hands of handset manufacturers and mobile operators, who will now have the chance to optimize the OS for their devices.

Microsoft is using the red Windows Phone theme as the official theme color for Mango. “The Mango update for current Windows Phone handsets will be ready this fall, and of course will come pre-installed on new Windows Phones”, said Microsoft’s Terry Myerson in a blog post on Tuesday.

The final build of Mango is 7720 according to Matthijs Hoekstra, a Windows Phone developer evangelist at Microsoft.

The software giant unveiled Mango in May with a focus on improving communication, app multitasking, and Internet browsing. Mango includes new features like Threads (a fusion of text, IM and Facebook Chat), a web-based marketplace for apps and a mobile version of Internet Explorer 9 with advanced HTML5 support.

We previously informed you Acer unveiled Windows Mango Device at Computex 2011. A Fujitsu branded Windows Phone will be made available in Japan by the end of August according to recent reports. The device will launch with carrier KDDI. The software maker also revealed a new Samsung Windows Phone that appears to resemble the company’s Galaxy S II Android model. Windows Phone Mango is expected to include front facing camera support. Microsoft has not yet officially detailed the hardware specifications for Mango devices.

Apple and Google’s mobile operating systems, iOS and Android, are still the dominant forces in the market, though some predict that WP7 will gain traction in the next few years.