Microsoft is making available to testers the first public preview of Windows 10 mobile, a version of Windows 10 ultimately designed to run on ARM-based Windows Phones and ARM- and Intel- small tablets and phablets.

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The preview bits for certain Windows Phones are available for download by those who have signed up to be Windows Insider testers. The release of the Windows 10 mobile preview follows by just over a week the release of the first public preview of touch-first Office for Windows 10 desktop users.

This first preview is available for installation only on certain Windows Phone devices, specifically Lumia 630, 635, 636, 638, 730 and 830 devices, according to Microsoft officials.

Microsoft officials promised at the end of January that the company would deliver the Windows 10 mobile preview in the month of February, but wouldn't provide an exact target date. Those officials showed off a glimpse of some of the capabilities that would be part of Windows 10 mobile -- beyond what would be in the first public preview -- during Microsoft's Windows 10 event in Redmond on January 21.

Spartan comes later

Officials said not to expect the new, lighter-weight Microsoft browser, codenamed "Spartan," to be part of the first public preview of Windows 10 mobile, though they briefly showed it off to those who attended the January 21 event. Microsoft plans to add Spartan, which will be the default browser for Windows 10 desktop and mobile, to the preview bits some time in the coming months.

Microsoft execs also said to expect Microsoft to build into the Windows 10 mobile SKU enhanced Cortana functionality, as well as updated and free versions of a number of its mobile applications, including Outlook, Calendar, Photos and Maps. (The updated Outlook for Windows Phone will look like the new Outlook apps for iOS and Android -- which are so far little more than Acompli email clientsfor those platforms.)

There will be new touch-first Office apps for Windows Phones running Windows 10 mobile, including Word, Excel and PowerPoint, plus an updated version of OneNote. Those applications will be bundled for free with Windows Phones and small Windows tablets running the Windows 10 mobile variant, and available for download on other versions of Windows 10, Microsoft officials have said.

Skype/SMS will be integrated directly into Windows 10 mobile (and desktop), as well, Microsoft officials have said.

Features that are included in today's first preview:

Full-size Background Image for Start

More Quick Actions in Action Center

Interactive Notifications

Significantly enhanced speech-to-text capability

More powerful Photos app

Microsoft officials have not said when to expect the company to launch Windows 10 mobile. I've heard from my sources that Microsoft is targeting this coming fall (October or so) to launch Windows 10, but I don't know whether that means all Windows 10 SKUs or not.

For those wondering, Microsoft also is planning to deliver an updated version of Windows 10 for desktop/laptop/tablet devices in February, but officials haven't provided a date.

Update: In a blog post about today's preview, Microsoft officials explained why the initial set of devices is limited to the specific set of Lumia devices that it is. From the post: