Retro Studios revealed some concept art for Donkey Kong Country Returns with the majority of the art being unlockable in the game itself.
The art comes from around the time very early in the game's development and when the art style had already been decided upon. Some of the art shows off ideas of levels borrowed from the Virtual Boy or the Mario vs DK handheld series.
http://kotaku.com/5731191/who-knew-d...-this-gorgeous
Link with more of the art:
http://conceptart.org/forums/showthread.php?t=201409