From the Portal Wiki

Each of the games in the Portal series went through many iterations and multiple design phases before their official release. At various stages of each game's development and release, various pieces of concept art were made public.

Portal

Handheld Portal Device Portal Gun concept art

GLaDOS GLaDOS concept art GLaDOS concept art

Portal 2

Aperture Science Handheld Portal Device

Handheld Portal Device Portal Gun concept art

Chell

Chell Chell concept art Chell concept art Chell concept art Chell concept art Chell concept art Chell concept art Chell concept art Chell concept art

Atlas and P-Body

Atlas and P-Body Concept Art Atlas and P-Body concept art Atlas and P-Body concept art Atlas and P-Body concept art Atlas and P-Body concept art Atlas and P-Body concept art Atlas and P-Body concept art Atlas and P-Body concept art Atlas and P-Body concept art Atlas and P-Body concept art

Forgotten Aperture Science Facility

Old Aperture Science Facility Old Aperture Science concept art Old Aperture Science concept art Old Aperture Science concept art Old Aperture Science concept art

Dioramas

Scrapped Aperture Science Diorama Exhibition Diorama concept screenshot Diorama concept screenshot

Box Art

Portal 2 concept box art Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design Portal 2 concept box art design