Lieutenant General Charles Davis testified to lawmakers from the Senate Armed Service Committee's subcommittee on tactical air and land forces that China may have used compromised American data to design and build the J-20 and J-31 fifth-generation fighter aircraft.
Similar accusations have been mentioned before, but this time it came from a very high-ranking US Air Force officer. The statements were part of criticism that defense contractors had not done enough to protect technology relating to the F-22 and F-35 stealth fighter programs.
Especially worrisome is the threat of cyber-intrusion from foreign entities. Lieutenant General Christopher Bogdan expressed doubts regarding security implemented. In response, companies such as Lockheed Martin have assured that security is well take care of and problems have been fixed.
While China often, and especially recently, gets singled out for cyber-intrusion, all major countries have active programs to gain as much intelligence from other countries. This includes countries that are officially allies or friends of each other.
For some reason, there's the tendency to associate every advancement China has made with espionage, especially in the media. There's the conviction that China couldn't possibly have done it by themselves and therefore must have stolen or copied technology from other countries.
The J-10 is still often referred to as the result of Israeli transfer of technology to China. To some people, the J-10 is basically the Lavi. Actually, there is lots of evidence why Israel could not play a significant role in the J-10, but people tend to ignore it.
The J-31 does seem to incorporate features from the F-35, but that does not mean China is using American technology. Often, engineers come up with the same solution to the same problem independently.
The J-31 is not even that similar in terms of design, despite a passing resemblance. The whole idea that China or any other country can simply develop new aircraft based on downloaded information is oversimplifying the complexity in developing a fifth-generation aircraft.
That doesn't mean cyber intelligence cannot help narrow down the path to take in solving a specific problem. It can speed up development somewhat, but countries will still have do all the really hard work on their own.