Part of project "Einstein", which Microsoft Research managing director Eric Horvitz is involved with, the company is working on advancing Cortana to improve on its personal assistant offering, prior to rolling out on other platforms. According to Reuters:

"Later, Cortana will be available as a standalone app, usable on phones and tablets powered by Apple Inc's iOS and Google Inc's Android, people familiar with the project said."

Currently, Microsoft has rolled out Cortana on compatible Windows Phones and in early builds of Windows 10. According to an interview between Horvitz and Reuters, Microsoft will offer a more advanced version of Cortana that could also bring in more automation and awareness, using data from artificial intelligence. From the interview:

"This kind of technology, which can read and understand email, will play a central role in the next roll out of Cortana, which we are working on now for the fall time frame."

It's yet another step in Microsoft CEO Satya Nadella's plan to offer Microsoft services and apps outside of the Windows ecosystem. We've already seen advances made with Office, releasing advanced versions on competitor hardware. It's believed Cortana could also prove useful in attracting consumers to Microsoft's ecosystem.

Source: Reuters