the Serbs shot down a US stealth jet in 1999 many thought that the pieces would have been flogged to Russia. However it is now claimed that the Serbs flogged the bits to the Chinese who have put out their own version last month.
Apparently the Chengdu J-20 has similarities to the American F-117 Nighthawk which was shot down during Nato’s aerial bombing of the country during the Kosovo war, by a Soviet-built SA-3 missile. Chinese agents travelled to the region where the F-117 disintegrated, buying up parts of the plane from local farmers.
Admiral Davor Domazet-Loso, Croatia’s military chief of staff during the Kosovo war told the Daily Mail
that the Chinese used those materials to gain an insight into secret stealth technologies... and to reverse-engineer them.’
It has taken a while to get the jet flying but reverse engineering can't be hurried.