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]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "by design":
   advisedly, calculatedly, consciously, contemplatedly, deliberately,
   designedly, in cold blood, intentionally, knowingly, meditatedly,
   on purpose, pointedly, premeditatedly, purposefully, purposely,
   purposively, voluntarily, willfully, with full intent, with intent,
   with malice aforethought, with purpose, wittingly