WASHINGTON — Silicon Valley has dreamed up hot air balloons, drones and constellations of mini-satellites to connect the world to the internet. Now Microsoft is adding its own moonshot to solve the digital divide into that mix.

The software giant plans to announce on Tuesday that it is harnessing the unused channels between television broadcasts, known as white spaces, to help get more of rural America online. In an event at the Willard Hotel in Washington, where Alexander Graham Bell demonstrated a coast-to-coast telephone call a century ago, Microsoft plans to say that it will soon start a white-spaces broadband service in 12 states including Arizona, Kansas, New York and Virginia to connect two million rural Americans in the next five years who have limited or no access to high-speed internet.

In an interview, Microsoft’s president, Brad Smith, said white spaces were “the best solution for reaching over 80 percent of people in rural America who lack broadband today.”

The United States has long grappled with uneven internet access. Many remote and less-populated areas have had particular trouble obtaining fast internet service because providers often cannot justify the cost of building out infrastructure to isolated locales with few people to serve.