DALLAS (AP) — American Airlines has agreed to buy up to 20 supersonic jets and put down a non-refundable deposit on the planes that are still on the drawing board and years away from flying.

Neither American nor the manufacturer Boom Supersonic would provide financial details Tuesday, including the size of American's deposit.

American, which also took options for 40 more Boom Overture planes, becomes the second U.S. customer for Boom after a similar announcement last year from United Airlines for 15 jets.

It has been nearly 20 years since the last supersonic passenger flight by Concorde, the British-French plane that failed to catch on because of the high cost — both for passengers and airlines.

Boom CEO Blake Scholl insists his company's plane will be different when it debuts in 2029, with tickets costing about $4,000 to $5,000 to fly from New York to London in about three and one-half hours.

"There are tens of millions of passengers every year flying in business class on routes where Overture will give a big speed-up," Scholl said in an interview, "and airlines will be able to do it profitably."

Boom says its plane will have a top speed of 1.7 times the speed of sound, or about 1,300 mph, and carry between 65 and 80 passengers.

Skeptics have questioned Boom's ambitious timetable, especially in light of the many years it has taken Boeing, an established manufacturer, to get planes or even retrofits to planes approved by the Federal Aviation Administration.

Notably, Boom does not yet have an engine manufacturer lined up. It is talking with Rolls Royce and others.

"With a supersonic jet, you don't design a plane, you design an engine first," said Richard Aboulafia, an aerospace analyst at consultant AeroDynamic Advisory. "This is just a collection of freehand drawings until that engine happens."

Boom says the plane will fly entirely on sustainable aviation fuel, often made from plant material, which is currently in short supply and very expensive.

Boom, which is based in Denver and plans to build the Overture in North Carolina, says the program will cost between $6 billion and $8 billion. The plane carries a list price of $200 million, although other manufacturers routinely give airlines deep discounts.

Last month, Boom announced changes to the plane's design to make it simpler and less expensive to build and maintain. The most striking change was going from three engines, including a different type on the tail, to four identical engines under the delta-shaped wings.

The market for four-engine planes is shrinking. The Boeing 747 is used mostly for hauling cargo now, and Airbus shut down production of the A380 in 2021. The vast majority of passenger jets flying today have two engines.

Four-engine planes "are that much worse from every standpoint, from economics to emissions," Aboulafia said. "Nobody wants more engines, the answer is fewer engines."

American Airlines said the supersonic plane will change travel.

"Looking to the future, supersonic travel will be an important part of our ability to deliver for our customers," said Derek Kerr, the airline's chief financial officer.

The union representing American's pilots questioned the timing of the airline's investment in planes that won't be available for several years at best. American has struggled this summer, canceling more than 9,300 flights since June 1 — more than double the cancellations at United, Delta or Southwest — according to FlightAware.

"Investing in today's operation should be management's sole focus," said Dennis Tajer, a spokesman for the union. "If there aren't any changes to how management schedules this airline and its pilots, these will just be supersonic cancellations."