According to Wiki, During the Development of Tunnel B1, the game was split into two section. Hovercraft and helicopter. At some stage, Ocean decided to take out the helicopter levels and make it its own game.
The game is pretty much a rail shooter. You kill enemies and make choices on direction to go at certain points. The choice is usually short route to the boss or long route with lots of enemies to fend off and bonus points to collect. Collecting 90 bonus points will allow to play a bonus stage at the end of each level. The bonus stages are usually just involve flying through rings and collect a life at the end. Miss a ring and it ends the bonus stage. Its not until near the end of the game I go out of my way to show these.
There are three difficulty options and each one gives you a different helicopter. An easy light weight one, a medium balanced one, and a hard heavy one with little armour. I choose hard since the heavy copter is much easier to control and also to avoid any crappy ending the game might throw at me. The enemies do not change much between difficulties.
The biggest issue with this game is that its so bloody dark. It makes it difficulty to see enemies and collectables and also, having the helicopter in the middle of the screen means I cant see any incoming fire. makes the game far more difficult that it needs to be.
The intro and ending are a little odd depicting you against robots where as the later bosses in the game are bugs. Also the game may have been rushed and missing content as after you kill the final boss, you are given more collectables while escaping which are of no use at all as the game is over!