This is actually really cool. Reminds me of that helicopter flash game.
I pretty quickly became so OP I could probably deal with an endless solid wall so I got bored and stopped. I think balancing, or even removing, the upgrading would make this a seriously addictive time waster. Adding more types of the explosive boxes seems like it could be fun too.
A bug I noticed is the bullets don't really go where you click because they are not affected by the horizontal camera movement. It doesn't matter once your bullets speed up, but in the early game it's pretty annoying.
I don't like the hold to shoot thing either, should be instant on click with a cooldown. Again, not actually a big deal since you mostly hold the button, but it doesn't feel good.