Hold Up + L + R, and press Start at the Konami screen.
Walk through the first mission without shooting or hitting any
targets. Your score should be 0 points. Defeat the Boss without
shooting its side gun turrets. Your score should be 2800 points.
When the asteroid level begins, press X to change weapons until
the unlimited napalm gun is selected. Survive this mission to keep
the unlimited napalm for the remainder of the game. Note: It cannot
be powered up.