[matthewdgreen]

It's a fascinating theory because according to the official documentation, the YB-49 didn't fly until later in the same year. Between the XB-35 and YB-35 and YB-49, there weren't supposed to have been nine operational units that could simultaneously fly in formation. (Although this picture shows exactly nine aircraft bodies that are in various states of conversion [0].) So for this not-totally-outlandish theory to be true, the U.S. military would need to have been aggressively developing and flying these planes beyond the level that they've publicly reported, and they would have to keep those unreported operations secret for an additional 73 years. Which is quite a conspiracy, and maybe even a more interesting one than "aliens".

But if you're in the mood for conspiracy theories, Wikipedia's description of the end of the XB/YB series is pretty surprising as well: "Only two months later, all Flying Wing contracts were canceled abruptly without explanation by order of Stuart Symington, Secretary of the Air Force. Shortly thereafter, also without explanation, Symington turned down a request from the Smithsonian for the Air Force to donate one of these big wings to its collection of pioneering Northrop aircraft. All remaining Flying Wing bomber airframes, except for the sole YRB-49A reconnaissance version, were ordered chopped up by Symington, the materials smelted down using portable smelters brought to Northrop's facility, in plain sight of its employees."

(The motivation for this is supposed to have been contract favoritism, but it looks a lot more exciting if you accept the idea that there was of still-secret-to-this-day unreported flight testing happening in 1947 :)

[0] https://en.wikipedia.org/wiki/Northrop_YB-49#/media/File:YB4...