Nice concept, roughly decent execution. I liked that the planes randomly switched directions. It added a bit of a challenge.
Few suggestions that come to mind are-
1. Let the planes shoot only after they are visible in the screen.
2. Start the game with slower airplanes. gradually increase the difficulty. (You could also assign specific behaviour patterns to each type of plane, kinda like the ghosts in Pacman)
3. Reduce the frequency at which the planes are firing, at least for some of the planes. If possible, you could add dynamic difficulty where they shoot fewer bullets when the player has lower health.
4. Add power ups where if you shoot certain types of planes, you get specific type of bullets or abilities. (or implement the power ups in some other way that you think works)
Overall, it's a nice game and has good potential. Nice work!