Boom Supersonic, the Colorado startup that’s at work at Centennial Airport on a 21st century vision of a supersonic airliner, received a ringing endorsement this week from the world’s largest air carrier, American Airlines.

In a joint statement, American and Boom announced that the carrier will make a nonrefundable deposit on as many as 20 of Boom’s Overture aircraft, with an option to buy an additional 40 airplanes.

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United Airlines, the world’s second largest airline by fleet size, announced last year that it would buy 15 supersonics from Boom and took options on 35 more.

In an interview with The Denver Gazette, Boom founder and CEO Blake Scholl said American’s commitment was even more significant than its initial agreement with United. “It shows we’re building something that makes economic, strategic and financial sense to airlines,” Scholl said.

Boom’s Overture would be a new version of the Concorde, the sleek jet that whisked passengers from Paris to New York between 1976 and 2003. But Boom’s version would benefit from 50 years of ensuing technology including lighter materials and computerized modeling. Concorde carried around 100 passengers and Overture is designed for 65 to 80 — and would be a foot shorter.

Japan Airlines was an early investor in Boom with a reported $10 million investment in 2017 and an option to purchase up to 20 aircraft.

Scholl underscored that the American and United orders were nonrefundable deposits, as opposed to the preorder Japan Airlines made. Adding up firm purchases, preorders and options, Boom has 130 possible orders, Scholl said. Executives have suggested $200 million as a possible price-per-aircraft. Boom declined to say how much United will deposit up front.

Meanwhile, some observers in the aviation and environmental communities were expressing doubt about the project based on its timetable and possible impacts on Earth’s upper atmosphere.

“It’s surprising to see American order supersonic aircraft that conflict with its own climate goals,” Dan Rutherford, aviation director at the California-based International Council on Clean Transportation, told The Denver Gazette in response to the new aircraft order. “It makes me wonder if they’ve done their due diligence.”

But Boom is roaring ahead on its plans in Colorado.

Those include final tests on a “Baby Boom” mini-supersonic that will be trucked to California’s Mojave Desert for its first test flight before the end of the year, Scholl said. Boom says it will also outfit a 70,000-square-foot testbed for a full-scale Overture prototype near Centennial Airport that would test the plane’s parts while a “Superfactory” plant is created in North Carolina. Boom said it hopes to carry passengers by 2029.

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“We take sustainability super seriously,” Scholl said, noting that Boom is being designed to fly 100% carbon neutral — no net carbon added to Earth’s atmosphere. “We believe in a future where people go more places, more often.”

Scholl said he sees Colorado as important to the craft’s vision. “I personally moved to Denver to put Boom there,” he said. “We knew we would have to get people to move, and we needed a place where they would be happy to live.”

Boom has 250 employees and contractors, Scholl said. “The company will be doubling its size over the next couple of years. We’re creating the future of flight, and my personal hope is that Denver gets more booms.”