Or the easier solution to the problem - invent a paint that can change its color roughly to match the sky. So if you are flying under a light-grey sky with a high overcast, the paint becomes light grey. Flying in a deep blue sky, the paint becomes blue. That gets you pretty close to ‘invisible’, although of course the plane still obscures things it flies in front of.
The higher tech method is to put a camera on the top of the plane, and a coating on the bottom with tens of thousands of ‘pixels’ of color-variant material. The camera looks up, a computer figures out what the observers on the ground would have seen if the plane wasn’t in the way, and the coating on the bottom creates an image to match it. That’s much closer to invisibility, although it suffers from parallax error - observers to the side of the ‘optimum’ location would see a slight distortion.
Or, just do what they did to make certain naval patrol bombers nearly invisible in WW2.
They mounted photometers on top of the plane, & quite a few variable output floodlights over it’s surface.
The floodlights were “tuned” to give off the same level of light that would be seen in the sky around the plane. This meant that, over a distance, an aircraft so equipped would be no brighter and no darker than the sky around it.
Such an aircraft is extremely difficult to pick out from it’s background, as two of the most important visual clues used by the eye to resolve images, relative brightness and contrast are denied.
How about that “stealth” bomber, huh? Truth is, it’s NOT invisible to radar - it just shows a much smaller return than an object it’s size normally would. In theory, if you knew what to look for on the scope, you could detect it (you would think a blip the size of a sparrow clocking 300mph would stand out). In practice, it gets lost among all the other returns which are bigger and brighter. So it’s not really invisible, just so hard to see it might as well be.
Likewise, the concept of being invisible to the eye is really an extreme form of camoflage. You could paint the plane to match the sky, so someone looking from below has a great deal of trouble seeing the plane - except the sky changes color. Maybe a color-changing paint? But such a thing might show up vividly from above, against the backdrop of the ground.
I’m sure there are things that can be done to make and airplane a lot harder to see.