WordNet (r) 3.0 (2006):

by design
    adv 1: with intention; in an intentional manner; "he used that
           word intentionally"; "I did this by choice" [syn:
           intentionally, deliberately, designedly, on
           purpose, purposely, advisedly, by choice, by
           design] [ant: accidentally, by chance,
           circumstantially, unexpectedly, unintentionally]