That compares to an expected 1.5 billion Windows devices by the end of this year, against 608 million using Android.
Android, which reached the market only in 2008, has risen fast to be the dominant smartphone platform, controlling two-thirds of that market. It has taken the No. 2 spot in the fast-growing tablet computer market.
The proliferation of the free software gives Google its edge on the search market - its key profit generator.
Worldwide shipments of personal computers fell by over 8 percent in the third quarter, the steepest decline since 2001, as more consumers flock to increasingly powerful tablets and smartphones for more basic computing.
Microsoft's Windows has dominated the personal computer industry for decades, but the company has struggled to keep up with shift to wireless, and in smartphones its market share is around 3 percent.