In order to get the Bomber Mansion you must use your bombs to kill yourself or die from one of the enemies. The professor will say something about training in the Bomber Mansion. Select 'No' when asked to continue, and on the main screen you will be able to use the Bomber Mansion to train. The Bomber Mansion is a training place, and when you complete all five levels you will receive a rank. Your score determines your rank. Your score will go down if you lost a heart (you can find them hidden in blocks) or if you place a useless bomb (a bomb that does not explode or kill anything).
Planet Bomber World
Get all ten Lightning Cards from the evil bombers, then defeat the Boss in level 5-4 (Warhorse). The Planet Bomber world will now be unlocked. In it, you will battle two new Bosses (Mujoe and Dr. Mechadoc).