Willful is the American spelling of the adjective meaning (1) done on purpose, or (2) intent on having one’s own way. Wilful is the preferred spelling in all the main varieties of English from outside North America. Both spellings appear about equally often in Canadian publications.
Wilful is the original spelling. Though willful has been around for many centuries, it did not become common until the 19th century. And, as the ngram below suggests, it became the preferred spelling in American English around 1950. Willful, meanwhile, has slowly gained ground in British English, but wilful is still far more common.
This ngram graphs the use of willful and wilful in American books published from 1800 to 2000.
The foreseeable and justified answer to willful provocation is protest. [Chicago Tribune]
But this willful, propagandistic distortion of American history is shocking. [New York Times]
Is nowhere, you sometimes wonder, free of wilful discrimination? [Guardian]
The great thing about rata vines is that with a little wilful manipulation these giants of the forest can be tamed to grow in a smaller environment. [New Zealand Herald]
A second charge of wilful exposure was formally dropped by police prior to his sentence. [iOL (South Africa]]
Dissenters suggest a compromise, that the lock clause be amended so that it applies only in cases of willful copyright infringement. [Globe and Mail]
We often hear of stories (or perhaps have our own tale to tell) about wilful tax cheats. [Globe and Mail]