Metal Slug: 1st Mission

Level select:
Successfully complete the game to unlock the "S-Continue" option that allows any level to be played again.

Invincibility:
Successfully complete the game under the normal or better difficulty setting and obtain a rank of 23/23. Note: Some falls will still be harmful.

Alternate female character:
Successfully complete the game to unlock an alternate version of the female character.

Play as Boss:
Successfully complete the game with a devil class rank under the very hard difficulty setting. The last Boss will be playable, with unlimited hit points but limited ammunition.

Alternate demonstration:
Go to the set up menu and change the button configuration so that you will jump with A. Have the game play by itself and the demo will do some unusual things.
Information in this section was contributed by Gbcn64master.

Hint: Super jump:
Jump on an opponent's head and immediately jump again to jump higher than normal.