United Airlines announced plans Thursday to buy 15 planes from airline startup Boom Supersonic in a move that could revive the high-speed form of air travel after the Concorde was wound down in 2003.

Under the commercial agreement, United would purchase Boom's "Overture" aircraft once the planes meet "United's demanding safety, operating and sustainability requirements" with an aim to start passenger travel in 2029, the companies said in a joint press release.

The announcement represents a potential comeback to a once heavily-touted method of travel, although some analysts expressed skepticism, particularly over the relatively speedy timeframe.

The agreement covers 15 planes and includes an option for United to obtain another 35 aircraft. The companies did not disclose financial terms.

"It's an interesting idea, but there are a lot of questions," said Michel Merluzeau, an expert at consultancy AIR, who estimates that developing a new commercial jet that passes muster with regulators could cost $10 to $15 billion.

"We need to be realistic about this," added Merluzeau, who sees 2035 or 2040 as a more likely target date for commercial service.

Merluzeau said it also was not clear whether United had agreed to any payments or if the announcement represented an intention to purchase.

Boom's plane is capable of flying at twice the speed of leading aircraft now on the market, with the potential to fly from Newark to London in three and a half hours and San Francisco to Tokyo in six hours, the companies said.

The jets will also be "net-zero" in carbon use because they will employ renewable fuel.