• Hypersonic aircraft start-up Hermeus announced a $100 million round of funding that it says will help complete development of its first prototype aircraft, with first flights planned for 2023.
• The company is developing aircraft that would travel at five times the speed of sound, or Mach 5.
• Hermeus' fundraising was led by venture capitalist Sam Altman, and joined by Peter Thiel's Founders Fund and In-Q-Tel, both new investors.

Hypersonic aircraft startup Hermeus on Thursday announced a $100 million round of funding that it says will help complete development of its first prototype aircraft and build out its fleet of high speed jets.

Hermeus' fundraising was led by venture capitalist Sam Altman, and joined by Peter Thiel's Founders Fund and In-Q-Tel, both new investors. The round included existing investors Khosla Ventures, Canaan Partners, Bling Capital, and Revolution's Rise of the Rest.

The company declined CNBC's request to disclose its valuation following the raise.

"Hermeus is pursuing an ambitious vision that seems impossible at first glance, but they pair it with an engineering culture and business roadmap that can actually bring it into reality," Altman said in a statement. In a tweet, he said: "i [heart] fast airplanes!"

The Atlanta-based company is developing aircraft that would travel at five times the speed of sound, or Mach 5. Founded in 2018, Heremeus has been developing its Chimera engine and Quarterhorse prototype aircraft, to demonstrate the capability and reliability of its approach.

Hermeus said the new funds will be dedicated to completing development of its first Quarterhorse jet, build three flight-capable Quarterhorse jets, and begin flight testing. Then, the company plans to move to development of its next aircraft, called Darkhorse.

"Quarterhorse is a sprinter, effectively the smallest possible airframe to flight test our engine, Chimera, across all modes of operation and Mach numbers. Darkhorse will be capable of sustained hypersonic flight and be able to carry cargo or payloads," Hermeus COO Skyler Shuford told CNBC.

The company unveiled its first, non-flying Quarterhorse prototype at an event in 2021, where Hermeus test fired the aircraft's engine. Hermeus last year said that it aimed to begin Quarterhorse flight testing in late 2022, but on Thursday said it is now on track to fly in 2023.

Hermeus' testing and development of Quarterhorse and Darkhorse are part of the company's plan to then build a hypersonic commercial passenger jet, called Halcyon.

The market for high-speed next generation jets has attracted a variety of new prototypes, such as Boom Supersonic's Overture or NASA's Quiet Supersonic Technology (or QueSST) aircraft, which Lockheed Martin is manufacturing. But the difficulty of building such vehicles is apparent through the typically high capital costs and development delays – risks emphasized by Aerion Supersonic, which closed down last year.

Hermeus' has an 110,000-square-foot factory in Atlanta and says it has completed over 100 engine test to date. Last year, Hermeus won a $60 million U.S. Air Force contract to begin flight testing Quarterhorse and validate its Chimera engine.