In the days when the Stealth Fighter (a misnomer, but that is another subject entirely) project was still top secret, there was a lot of speculation about what it was and what it was called.
When the F20 TigerShark was released, right after the F18 Hornet, there was some speculation that the Stealth Fighter was the F19. That was, not however, the case. The makers of the Tigershark had just requested that the plane be numbered 20 to represent how it was part of a new generation of fighter craft.
There was much speculation about what the F19 would like and do. It was widely believed that it would be shaped with soft, round curves and be in the shape of a flying wing. And, as the name fighter suggested, that it would have air to air capability.
The actual plane that was being developed, the F117, was not actually a fighter at all, having no capability for air to air weapons, and instead it had sharp, rigid angles. It did, however, have a flying wing shape.
So the F19 was the epoch making airpane that didn't exist. It did, however, inspire a great game, F19 Stealth Fighter, from Microprose.