We all know that scientific research on the same subject(s)_Stealth design in this instance_ goes on in different parts of the world in parallel, That is why this talk of copying makes no sense.. it is more like a race in R&D than anything else..
No, it was not. Parallel R/D means competitors starts with roughly the same information.
The US started it and we are at least one decade ahead of everyone else. With the F-117, we sort of 'gave' our competitors a considerable assist. Not in the sense that we invited them to observe our research, but in the sense that we showed the research paid off in tangible ways and those ways -- combat -- they never had to risk. They knew, at no cost to them...
- That the angled faceting technique worked and worked well,
- That the US dropped that technique in favor of curvatures due to advances in computing power to calculate the much more complex behaviors of waves on curvatures,
- That aerodynamics efficiency does not have to be reduced to work successfully with 'stealth'.
- That the use of absorber composites do not have to be comprehensive on the 'stealth' body,
And
- That there are tactically useful levels of radar returns from the 'stealth' body that require less rigors in design, thereby reducing development cost.
No one, from the Soviets to China to any major power, that have their own in-house aviation program spent a single pound, yuan, ruble, franc, or deutsche mark to find out those things. We did it all and in spectacular ways.
That did not happened with the Horten flying wing design. The 229 never made it into production, let alone combat. It was barely into its flight testing regime. The Hortens may have infused their design with iron oxide compounds, like the ones used on submarine snorkels and periscopes, to try to reduce the 299's vulnerability to radar, but current information does not support the speculation that the 229 was actually subjected to any significant open radar testing. In other words, the world knew nothing of the 299 until the end of WW II.
The flying wing was already common knowledge and exploration as a viable aircraft, not just a scientific study, was truly in parallel. So no one copied from anyone else at that time.
Like it or not, the charge that the J-20 came from the MIG 1.44 is a serious one.