Pearl Harbor: Zero Hour Cheats

At the mission selection screen, click the airplane shadow on the right side of the screen to unlock an extra level and three additional airplanes.

In mission six go near the top of B3 and you will see an island. Destroy the tree on the island and you will earn 500 points.

In almost every level you will find isolated palm trees. Shooting these trees down gives you 500 skill level points, relaying the message "Enemy James Brown's Favorite Tree, +500." I can only assume these were all developers (the names of the "enemy ***'s"). Most levels have more than one, and they are all single trees, not the bunches and thickets of trees throughout the map.