The US Air Force has already tested its next-generation fighter plane – that may even have flown without a pilot.

Key details of the test are being kept secret and there’s been no confirmation of whether the prototype is an advanced UAV or a more traditional crewed fighter.

But Dr Will Roper, Assistant Secretary of the Air Force for Acquisition, Technology and Logistics, announced to the Air Force Association’s virtual Air, Space & Cyber Conference that a prototype airframe for the Next Generation Air Dominance (NGAD) program had already undergone flight testing.

“We’ve already built and flown a full-scale flight demonstrator in the real world,” he told Defense News, “and we broke records in doing it. We are ready to go and build the next-generation aircraft in a way that has never happened before.”

Dr. Roper refused to offer any details of the super-secret aircraft

He added: “A lot of the mission systems that we require for Next-Generation Air Dominance have been flown on test articles, so they are coming along very well, and digital engineering seems to accelerate everything.”

But Roper declined to reveal any details on how many of the new prototype aircraft had been flown or where they had been manufactured.

He also refused to divulge any aspect of the aircraft’s design, speed or armament. He wouldn't be draw on whether the new plane had a human pilot or would be controlled by an artificial intelligence.

The F-35 is expected to remain in service until at least 2044

But the Next Generation Air Dominance program is a planned network of interconnected assets so that combat aircraft, unmanned craft and cyber warfare systems can work together to overwhelm any opposing force.

The Drive notes that there has been a marked increase in test flights over the American South-West, and speculates that at least some of this new activity could be connected to the NGAD project.

The announcement at the conference shocked defence experts, who say they hadn’t seen any hint of the ultra-secret tests. “This certainly helps explain where the NGAD funding was going,” Todd Harrison, director of the Aerospace Security Project at the Centre for Strategic and International Studies, told Breaking Defense.

“One of the things we noted in our recent budget analysis," he added, "is that the program was already at a burn rate of around $1B per year, even though the acquisition strategy had not been finalised.”

He added that all this spending would need to be justified by defence chiefs, given that the F-35 Lightning II is nowhere near the end of its service life.

“Despite the existence of a prototype, the Air Force is still going to need to make the case that it is time to switch to a new fighter when the F-35A has still not reached full rate production.”