United Airlines announced Thursday it will buy 15 planes from Denver-based Boom Supersonic, part of investments the carrier has made in aircraft of the future.

Under the terms of the agreement, United said the purchase will be made once Boom's Overture airliners meet its "safety, operating and sustainability" requirements. United also has the option of purchasing an additional 35 aircraft under the terms of the deal.

The company's planes have yet to be approved or built.

Once operational, the planes would be capable of flying at Mach speed 1.7 — twice as fast as today's fastest commercial jetliners. That would mean a flight between New York and London would take 3½ hours instead of the current 6½ hours, according to Boom Supersonic. The jets would be all business class and carry 65 to 88 passengers. Service could begin by 2029.

The planes also are expected to run on sustainable aviation fuel, making them "net-zero carbon from day one," according to United's announcement.

"United continues on its trajectory to build a more innovative, sustainable airline and today's advancements in technology are making it more viable for that to include supersonic planes," United Chief Executive Scott Kirby said in a statement that accompanied the announcement. "Our mission has always been about connecting people and now working with Boom, we'll be able to do that on an even greater scale."

Financial terms of the agreement weren't immediately available Thursday.

United said it is eyeing possible routes that include Newark to London, or San Francisco to Tokyo, which could be done in six hours compared with the current flight time of more than 10 hours.

"The world's first purchase agreement for net-zero carbon supersonic aircraft marks a significant step toward our mission to create a more accessible world," Blake Scholl, chief executive of Boom Supersonic, said in a statement.