The buildup is a worry for neighbours uneasy about China flexing its military muscle, especially in territorial disputes with Japan in the East China Sea and with Vietnam and the Philippines in the South China Sea.

“Just like the U.S. F-22 and F-35 fifth-generation fighters, the J-20 and J-31 will complement each other during future operations,” Bai Wei, former deputy editor of the weekly Aviation World, told the Global Times newspaper.

“The J-31 is almost certainly designed with the intention to have the potential of operating on aircraft carriers, judging from its enhanced double-wheel nose landing gear and two big tail wings, which help increase vertical stability,” Bai said.

China needs both the heavier J-20 and more nimble J-31 to defend its air space, Bai said.

The J-31 is a mid-sized fighter using Russian-made engines which will later be replaced by Chinese engines, the Global Times reported.

“The big Achilles heel for Chinese aerospace generally, and particularly for both of these two programmes, is engines,” Roggeveen, a former analyst for Australian government intelligence and editor of the Lowy Institute’s blog LowyInterpreter.org.

“They still rely very much on foreign technology, and their progress on developing domestic high-performance engines for combat aircraft has been frustrating and slow,” he said.

While the J-31 and J-20 will add to China’s offensive as well as defensive capability, “it will take many, many years” for them to enter service with the air force, Roggeveen said.