Windows Phone vice president Joe Belfiore introduced Cortana on stage at the technology titan's annual developers conference.
"Cortana is the first truly personal digital assistant who learns about me and the things that matter to me most, and knows about the whole Internet," Belfiore said in a presentation opening the gathering in San Francisco.
Cortana responds to conversationally spoken requests or commands, using insights gleaned from calendars, contact lists, online searches and other smartphone sources to respond in a manner akin to a real-life aide, Belfiore demonstrated.
It comes as a long-awaited counter to the Siri virtual assistant on Apple mobile devices and Google Now capabilities in Android tablets and smartphones.
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Cortana will be in a test, or beta, mode when it becomes available in a Windows Phone 8.1 software update to begin rolling out in the United States in coming months.
The new version of Windows Phone 8.1 should be available on new phones beginning in late April or early May, according to Belfiore.
Microsoft met with real-life personal assistants while designing Cortana, which is powered behind the scene by search engine Bing, he said.