The four aircraft will include the currently in-development Next Generation Air Dominance (NGAD), as well as the F-35, F-15EX and F-16.

This is according to USAF’s chief of staff, General CQ Brown, who made the comments at a McAleese and Associates virtual conference.

Notably this list doesn’t include the F-22 Raptor, which will be retired as its replacement, NGAD, goes into service.

Related: Lockheed Martin boosts classified aircraft project sales by $135m

The NGAD fighter will fly alongside the F-35, with the latter “the cornerstone, [the] F-15EX as we come in, and then we'll have F-16 for a while as well,” Brown said.

NGAD is a family of systems, with a sixth generation fighter to replace the F-22 at its centre (commonly referred to as F-X). Last September it was revealed that the fighter had been, designed, developed and tested in the space of a year and flown.

USAF’s top acquisition official, Will Roper, told Defense News: “We’ve already built and flown a full-scale flight demonstrator in the real world, 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.”

Little is known about the aircraft, but USAF just dropped a clue: concept art of a fighter jet being built under the NGAD programme. The image, seen at the top of this article, appears in USAF’s recently released biennial report for acquisition. Read the full report here.

More sixth generation fighters: Find out about US Navy's own NGAD fighter, F/A-XX

The Air Force is determining the right mix of fighter jets through a tactical air study that all the US services are participating in, to look “across the board, [at] all of our combat aircraft, our attack, our fighter portfolio.”

“I'm really looking for a window of options, because the facts and assumptions based on a threat will change over time,” he said. “But I want to get us shaped in a direction, because right now we have seven fighter fleets. Okay, my intent is to get down to about four.”

Last year, USAF leaders hinted that they might buy the F-15EX not just to replace older F-15Cs but also the 218 newer F-15E Strike Eagles.

Brown said the service is still considering what will replace the F-16, “whether it's additional F-35, or something else into the future. But I don't need to make that decision today. That's probably six, seven, eight years away into the future.”

The F-22, manufactured by Lockheed Martin, made its combat debut in Syria in 2014 but has never shot down an enemy plane in combat.

US allies have expressed interest in purchasing F-22s, but laws prohibit its export.

USAF had originally planned to buy 750 F-22s. In 2009, the programme was cut to 187 operational production aircraft due to high costs, a lack of clear air-to-air missions due to delays in Russian and Chinese fighter programmes, a ban on exports and development of the more versatile F-35.

Contractor Lockheed Martin delivered the final F-22 in 2012.

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