United Airlines announced a plan to buy 15 supersonic jets from a still-in-testing startup Thursday, returning the hope of supersonic travel to commercial customers 18 years after the last took the skies.

We should say upfront that the company that United partnered with, Boom Supersonic, isn’t exactly a household name and hasn’t actually built this lovely machine. Boom Supersonic has only built a proof-of-concept plane called the XB-1, which is much smaller and hasn’t even flown yet. Still, United Airlines seems to believe in the dream of these sharp-looking fast planes taking to the sky. They’re called Overtures and United has an initial order for 15 jets, but if these sharp flyboys cut the mustard for United in terms of safety and reliability, there’s 35 more jets on the table for purchase.

The Overture is planned to zoom along at 1.7 Mach speed and should be net-zero carbon from day one, United said in a . The plans will be optimized to run on sustainable aviation fuel. The planes should be in United’s hands by 2025 with plans to take to the skies with passengers by 2029.

Imagine flying from Newark to London in just three and a half hours, or San Francisco to Tokyo in just six. United also says that the Overtures will have “...features such as in-seat entertainment screens, ample personal space, and contactless technology.” Much like the Concorde, I’d expect seats on these Overtures to likely come at a premium.

The last commercial jet hit Mach speeds was the Concorde, which flew from 1976 to 2003. That supersonic jet could climb to 60,000 feet and speed along at 1,500 mph, which is Mach 2 — more than a thousand miles an hour faster than a 747. But the Concorde didn’t last. Seats on the supersonic jet were both cramped and expensive. In the early 2000s, a ticket from New York to Heathrow cost around $7,000, with a round trip ticket costing $10,000, according to the . The Concorde was also notorious for being tricky to fly and left plenty of safety accidents in its wake. It was also dirty, using monstrous amounts of fuel to get up to its incredible speeds. Most countries didn’t want the Concorde flying over their skies due to the sonic boom it created as well; a problem United doesn’t address in its press release.

It seems too good to be true at the moment: a supersonic plane that runs on green fuel. And to be honest, it probably is. Startups aren’t exactly known for always delivering on their wild promises. (Remember when Elon Musk said his Big Fucking Rocket could be used for city-to-city travel?)

The Concorde couldn’t make it work, but maybe the 20th Century just wasn’t ready for such high tech, high speed travel. Solving both the problems of dirty and slow air travel will be a daunting task and we will have to wait until the end of the decade to find out if the Overtures fit the bill.