Boom Supersonic's XB-1, the Symphony Engine, and the Startup Trying to Build a Supersonic Airliner Nobody Else Would Touch

On January 28, 2025, a privately built demonstrator called XB-1 became the first civilian-built supersonic jet in American history to break the sound barrier over the continental United States - no government program, no Air Force, just a startup called Boom. It was a genuine engineering achievement, but it was also the easy part of a much harder problem: building a supersonic airliner from a blank sheet of paper. Whether that airliner, Overture, ever carries passengers will come down almost entirely to one thing - an engine called Symphony that Boom is building itself because the world’s established engine makers declined to.

Why Did Supersonic Passenger Travel Disappear?

To understand why this is so hard, you have to start with Concorde.

The Anglo-French Concorde carried paying passengers from 1976 to 2003, cruising at Mach 2 and crossing from New York to London in about three and a half hours. It was an engineering triumph and a commercial failure. It burned enormous amounts of fuel, and its sonic boom was loud enough to rattle windows - so loud that the United States banned overland supersonic civil flight in 1973.

That ban stranded Concorde on a handful of transatlantic ocean routes. Only 20 aircraft were ever built, and just 14 flew commercially. After one crashed outside Paris in 2000, the economics never recovered.

For roughly two decades after Concorde retired, the fastest way across the ocean actually got slower. Aviation traded speed for efficiency, range, and cost per seat - a good trade, mostly. But the speed ceiling came down and stayed down.

Why Didn’t Boeing or Airbus Build the Next One?

The idea never died. It fell into a gap.

A supersonic airliner was too commercial for the military to fund, and too expensive and risky for Boeing or Airbus to bet the company on. In a global industry worth more than $300 billion a year, this one problem sat orphaned because it required someone willing to lose a lot of money for a long time on a maybe.

That is the gap Boom Supersonic walked into. Founded in 2014 in Colorado by software entrepreneur Blake Scholl, the pitch was simple to state and brutal to execute: build a 65-to-80-seat airliner called Overture, cruise it around Mach 1.7, and fly it over water first to sidestep the overland boom ban on day one.

What Has Boom Actually Built So Far?

The honest answer is XB-1 - and it’s important to separate what Boom has done from what it has promised.

XB-1 is a demonstrator: roughly one-third scale, two seats, three small engines. It was never meant to be the airliner. It’s a flying test article, the way you build a small thing to prove you can build the big thing.

And it ran late. First flight was originally targeted for around 2020; it didn’t actually fly until March 2024 - a multi-year slip on the comparatively easy part. That delay is worth remembering when weighing the timeline for the hard part.

But it did fly. Over about 11 test flights, the team pushed it faster, and on January 28, 2025, with test pilot Tristan Brandenburg at the controls, XB-1 went supersonic - three times in a single flight, reaching roughly Mach 1.1. They repeated the feat a couple of weeks later.

What Is “Boomless Cruise”?

During testing, Boom demonstrated something genuinely useful. Under the right atmospheric conditions, XB-1’s sonic boom never reached the ground - it refracted upward in the atmosphere and bent away before hitting the desert floor. Boom calls this “boomless cruise.”

The underlying physics isn’t new; it’s a decades-old phenomenon called Mach cutoff. What’s new is flying it deliberately with a real aircraft and measuring that no boom landed. That matters, because if you can reliably fly supersonic without dropping a boom on the people below, the entire overland-ban conversation changes.

Is the Overland Supersonic Ban Going Away?

Possibly. In the summer of 2025, an executive order and FAA action moved to reconsider the 52-year-old rule banning civil supersonic flight over land.

The FAA was directed to study replacing a hard speed limit with a noise-based standard: fly as fast as you like, as long as no boom reaches the ground. If that change holds, it rewrites the business case - Overture would no longer be confined to ocean routes, removing the single biggest constraint that killed Concorde’s route map.

Why Is the Engine the Hardest Problem?

Here is where the story gets difficult, because the engine is the crux of everything.

You cannot buy an off-the-shelf engine that does what Overture needs. Nobody makes a civil supersonic engine anymore. Concorde’s engines were military-derived and used afterburners, which are fuel-hungry, loud, and wrong for an airliner. Overture needs an engine that can supercruise - sustain supersonic speed without an afterburner - run on sustainable aviation fuel, and meet modern takeoff-noise rules.

Boom initially planned to partner with an established engine maker. Rolls-Royce studied it, then walked away in 2022, saying commercial supersonic wasn’t a priority. One by one, the major engine makers - the companies with the deepest jet-engine expertise on the planet - said not us, not now.

So Boom made an enormous decision: build the engine itself. It announced Symphony, a clean-sheet, medium-bypass turbofan designed for supercruise, developed with a set of smaller specialist partners.

This is the pivotal point. Building an airframe is hard; building a jet engine is a different order of hard. Engine programs routinely cost billions and take a decade or more, and they are where aerospace ambitions go to die. The airframe is arguably the easier half of Overture - and Boom took on the harder half from scratch, precisely because the experts passed. It’s either the boldest move in the story or the one that sinks it, and that won’t be clear for years.

Can Boom Afford to Finish?

Money and time are the second problem. Boom has raised capital, built a factory in North Carolina, and collected pre-orders - United and American Airlines have both put down deposits.

But pre-orders are conditional. They’re options, not cleared checks, and they evaporate the moment a program slips or the economics stop working. Getting a certified airliner into service will require billions more than Boom has raised. The company targets passenger flights around the end of this decade; given that the demonstrator alone ran years late, a realistic observer should pencil in longer - and remember that plenty of well-funded aerospace startups have run out of money before their aircraft ever left the ground.

Do the Economics Actually Work?

This is the same ghost that haunted Concorde. Supersonic flight burns more fuel per seat than a modern subsonic jet. That isn’t a Boom problem - it’s physics. Pushing air out of the way at Mach 1.7 costs energy.

The bet is that enough passengers will pay a business-class premium for time to fill 65 to 80 seats, repeatedly, across enough routes, to make the math close. That’s a market bet stacked on an engineering bet stacked on a regulatory bet. Every one of them has to land.

The Balanced Verdict: Watch the Engine

XB-1 is a real achievement - a privately built jet went supersonic over America for the first time, something many people said a startup couldn’t do. The boomless-cruise data is legitimately useful, and if the overland regulatory shift holds, it removes Concorde’s biggest killer.

But the demonstrator was the easy part, and it was late. The airliner is far harder. And Symphony is the hardest part of the hardest part, built in-house because the people who know engines best took a pass. The timeline will almost certainly stretch, the capital required is enormous, and the market still has to show up.

The honest way to hold it: root for the physics, respect the difficulty, and watch the engine - not the renderings, not the airline orders. When Symphony runs on a test stand, then runs reliably, then hangs on a flying airframe, that’s when you’ll know whether Overture is an airplane or a beautiful idea. Everything else is downstream of it.

For fifty years, the fastest way across the ocean got slower. Somebody finally decided that ceiling didn’t have to stay put - and whether or not Boom succeeds, the next attempt will learn from this one.

Key Takeaways

• XB-1 broke the sound barrier over the US on January 28, 2025, becoming the first civilian-built supersonic jet in American history to do so - piloted by Tristan Brandenburg at roughly Mach 1.1.
• Overture, Boom’s planned 65-to-80-seat, Mach 1.7 airliner, faces three stacked risks: engineering, funding, and market demand.
• The Symphony engine is the decisive factor - Boom is building it in-house after Rolls-Royce and other major engine makers declined in and around 2022.
• A 2025 FAA move to replace the 1973 overland ban with a noise-based standard could remove Concorde’s biggest commercial constraint.
• Boom has deposits from United and American, but pre-orders are conditional and the program needs billions more to reach certified service, targeted around the end of the decade.