The nose of ZeroAvia’s Q400. The company is partnering with Alaska Airlines to retrofit the aircraf with ZeroAvia’s powertrain. (GeekWire Photo / Lisa Stiffler)

ZeroAvia announced Monday that it has raised its largest round of funding to date. The company didn’t disclose the specific sum, citing legal reasons, but previous rounds include a $72 million Series B round and a $56 million Series A round.

ZeroAvia is developing hydrogen-electric propulsion systems for aircraft. The new investment will help finance the company’s pursuit of certification for its first engine.

The round was co-led by Airbus, Barclays Sustainable Impact Capital and NEOM. Bill Gates’ Breakthrough Energy Ventures, Horizons Ventures, Alaska Airlines, Ecosystem Integrity Fund, Summa Equity, AP Ventures and Amazon Climate Pledge Fund also participated.

The startup is based in California and the United Kingdom, and has a growing connection to Washington state. It has R&D operations in Everett and is partnering with Alaska Airlines to retrofit a retired plane with its propulsion system.

ZeroAvia has completed the first stage of flight testing of the prototype of its ZA600 powertrain. It hopes to begin offering engines capable of powering a 20-seat aircraft in 2025.

Its engines use hydrogen fuel cells to generate electricity, which powers electric motors to turn the aircraft’s propellers. The system produces water as its byproduct.

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