It plans to have a billion such devices running Windows 10 with the first two or three years of the release of the new software, making the Windows platform bigger than Apple's iOS and Google's Android combined, officials here at Build said.

Before a crowd of enthusiastic software developers, Microsoft showed off a new version of its software development platform, Visual Studio, that can take software code written for Android phones and iPhones, and convert it to a Windows app with little or no rewriting by the developer. That could make it easy for Microsoft's app store to match the app offerings of its bigger rivals, because developers will be able to convert their apps to Windows with little additional expense, meaning the question moves from being "why" develop for Windows phones given it has so little market share, to "why not?"

Apps converted over to Windows from other platforms would still be able to take advantage of core Windows features such as the location and notification services, Microsoft officials said. With a little more programming, they could also take advantage of other Windows features, such as the Xbox Live achievements system, that gives gamers points for playing games.

Microsoft also showed off a simulated version of upcoming Windows 10 phones, that will be capable of running in PC mode when they're attached to a bigger display. The so-called Windows Continuum system, which had previously been announced for tablets that could double as desktop PCs, will require new, dual-screen graphics hardware in the phone, and so won't be available for existing Windows phones as part of the free upgrade to Windows 10 that Microsoft if offering existing customers.

Microsoft's Build developers conference in San Francisco

In a demo of Windows Continuum for the phone, Microsoft showed off a cut-down version of PowerPoint running on a smart phone, that converted to a regular version of PowerPoint when the phone was attached to a screen. Such a feat was possible, Microsoft said, because it's the same version of PowerPoint running on phones, tablets and PCs anyway: so called "Universal" apps are the same program running on all the different devices, only with their output customised to the type of device.

The technology could be useful for companies, that could offer screens, keyboards and mice to visitors, who then would be able to work using their phone as if they had brought their PC with them, officials said. It could also be useful in developing "mobile first" countries, where people can afford to buy phones but not PCs.

John Davidson is at the Build developers conference in San Francisco as a guest of Microsoft