The list of airlines that want to bring back supersonic travel is growing.

Boom Aerospace, an ambitious aerospace start-up, says it has convinced five airlines to buy into its vision for developing a airliner that flies faster than the speed of sound.

Blake Scholl, founder and chief executive of Denver-based Boom, said during a presentation at the Paris Air Show Tuesday that the five carriers have reserved their spots for 76 of Boom’s successor to the Concorde. That aircraft was taken out of service in 2003 after 27 years of commercial flights at more than twice the speed of sound.

With a planned speed of 2.2 times the speed of sound or 1,451 mph, the project’s lofty goals include sharply cutting flight time: New York to Paris to three and a half hours, San Francisco to Tokyo in less than five and a half hours and Los Angeles to Sydney in just seven hours.

Only one airline interested in Boom, so far, has made itself known. Sir Richard Branson’s Virgin Atlantic Airways in 2016 has an option to take the first 10 aircraft the company builds.

Related: Virgin Atlantic faces a test: Is it enough just to be cool

Scholl won’t say who the four unidentified “world airlines” are. He promised the individual carriers would identify themselves at a later point to explain how a new generation of supersonic travel fit in their business.

“The airlines that are placing reservations with Boom now are putting real money against them, not small amounts, significant amounts of cash,” he said during a presentation at the Paris Air Show.

Two-steps to supersonic flying

Boom is advancing on a two-stage process to revive supersonic commercial air travel. Its first step is to develop a small aircraft called the XB-1 “Baby Boom” to validate many of the technologies for flying at Mach 2.2. Boom will first need to prove these work if it is to go forward with a full-size 45 to 55-seat airliner.

That small two-seat model begins assembly this year and flies late in 2018, Scholl said.

Scholl said that computer simulations have significantly sped up its initial process of finding the the right design of the full-size Boom jet.

“We’ve tested about a 1,000 design variations…most of them aren’t very good,” Scholl said. “But we’ve had a few that are.”

Related: Boom lands $33 million to test supersonic aircraft

To date, the company has raised $41 million, which Scholl said will get the XB-1 airborne for the first time.

But Boom is going to need considerably more capital to turn its initial airborne technology trials into a full-fledged commercial aircraft program.

Japan, Russia and China have all offered significant state support as part of the development of new homegrown airliners. The barriers for a new commercial aviation start-up, especially a private one, are enormous.

“I think healthy skepticism is only fair,” Scholl said. “This is not a simple project.”

CNNMoney (Paris) First published June 20, 2017: 8:25 PM ET

Electric-bus maker

Proterra

may be benefiting from President Donald Trump’s decision last month

to pull the US out of the Paris climate agreement

. The US-based company, which has factories in South Carolina and Southern California, just received

a $55 million round of equity funding

led by Al Gore’s Generation Investment Management and

BMW’s i Ventures

. In fact, Proterra represents the Bimmer venture group’s first-ever investment in heavy-duty transportation. That round, Proterra’s sixth, follows up a funding round earlier this year worth about $140 million.

More important, Trump’s decision

may have spurred more orders for the buses

, which can go between 200 and 350 miles on a full charge,

Forbes

says. Many cities took issue with Trump’s decision and, as a result, municipal leaders decided to

trade in

more

diesel

and natural-gas powered buses for the electric variants, which cost about $750,000 each.

Proterra, whose headquarters are in the Silicon Valley, says there are about 400 buses on order, with cities such as Dallas, Nashville, Philadelphia, and Seattle taking the plunge. The company more than tripled its sales in 2016, and is tripling production this year. Proterra estimates that it has a 60 percent market share of the North American electric bus market. The company is also working on an autonomous bus program with the University of Nevada, Reno.

Last year, Proterra made news in these pages by

releasing about two-dozen of its fast-charging patents

to the general public. Because of such fast-charging capabilities, Proterra says its buses can go as far as 700 miles in a single day.

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Starting this fall, the University of Michigan will start

offering students rides

on fully autonomous electric shuttles. The two 15-passenger vehicles will operate on U of M’s north campus in Ann Arbor, Michigan. The project is part of Mcity,

the faux town set up in Ann Arbor

that was designed as a testbed for

autonomous vehicles

. Mcity itself is a U of M-led collaboration among several public and private enterprises.

The two shuttles are manufactured by French company Navya. The Arma model looks similar to the

Local Motors Olli

, no surprise as both vehicles serve essentially the same sort of purpose. Arma shuttles have been

operating in Sion, Switzerland,

for more than a year now, proving that the technology works well enough in limited applications. We even had a chance

to ride along in one

at this year’s CES.

The Arma uses a 33-kWh LiFePO4 battery and has a run time of about nine hours, good enough for a full day’s duty on Michigan’s campus. Recharging can be done both with or without wires, through wireless charging does take significantly longer. The run speed is 15 mph, though an Arma can reach a blistering 28 mph with all tethers removed. The shuttles use Lidar,

GPS

, WiFi, and traditional cameras to navigate around a pre-set route.

No driver, no problem! The @navyatech arma can take you where you need to go! http://pic.twitter.com/ogQ5arZyCq

— Mcity (@UMichMcity) June 21, 2017

In this case, the two shuttles will run on a nonstop loop from the Lurie Engineering Center and the North Campus Research Complex. Both shuttles will cover the route in 10 minutes, and there is no charge for passengers. The project will be used to gauge reactions from both passengers and surrounding pedestrians, cyclists, and drivers.

Mcity and Navya seem to be making a big leap with their partnership. The French company just announced

a new manufacturing facility

in Southeast Michigan. The facility will produce more Arma shuttles, presumably for use at locations around the country.

Related Video: