As promised earlier this week, Microsoft is making available to its Windows Insiders testers the second test build of its Windows 10 Mobile release for Windows Phones on April 10.

@leonzandman

This build will work with nearly all the existing Lumia Windows Phones, thanks to a partition-stitching fix the team developed. However, this new test build still will not support the Lumia Icon or Lumia 930 phones, due to screen-size/scaling issues, officials said. It also doesn't yet support Windows Phones from any of Microsoft's phone OEMs.

Today's new build does include a test build of Microsoft's "Spartan" browser, which will be the only browser supported on Windows 10 Mobile devices. In this test version of Windows 10 Mobile, however, Spartan exists side-by-side with IE 11.

Microsoft also recently released a new test build of Windows 10 Desktop that included Spartan, too -- though on the versions of Windows 10 for Intel-based PCs, laptops and larger tablets, Microsoft will continue to include Internet Explorer 11, too, for backward compatibility reasons.

New test builds of universal versions of Outlook Mail and Outlook Calendar are included in today's Windows 10 Mobile preview. It also includes new Phone, Maps and Messaging apps, as well as a redesign of the People App, according to today's blog post about the release.

Microsoft has removed the Office Mobile hub in this build in anticipation of releasing the new separate Word, Excel, PowerPoint and OneNote universal apps for phones "in the coming weeks." That means testers won't be able to use any of these files in this build.

Microsoft released its first public test build of Windows 10 Mobile in February.

During a guest appearance on the Windows Weekly podcast (where I'm a co-host) earlier this week, Windows 10 Insiders program manager Gabe Aul said that the Windows 10 Mobile release ultimately will support small ARM- and Intel-based tablets, but only brand-new devices that haven't yet been released to market. That's why the Windows 10 Mobile test release currently works on Windows Phones only.

Microsoft officials have said they plan to launch Windows 10 this summer. According to sources, Microsoft will continue to deliver regular feature and bug-fix updates to Windows 10 Insiders and consumers after Windows 10 is released to manufacturing, along with a relatively major update to the Windows 10 operating system later this fall.

Update (2:30 pm ET): Those attempting to download the latest Windows 10 Mobile build, which was set to be available at 10 a.m. PT/1 p.m. PT today may be encountering issues. Microsoft hit a database provisioning error, which may result in a two-hour delay in the availability of the new bits, according to an update to the Microsoft blog post announcing the new preview.