The only free and comprehensive online etymological dictionary of the Spanish language
bien (Adverb, Adjective) "well"
Early 12th cent. From Latin bene 'id.,' from Old Latin *duened "good." From Proto-Italic *dweno- "good" (see bueno).
Indo-European
Romance
Asturian bien, Portuguese bem, Galician ben, Catalan bé, ben, French bien, Italian bene, Aromanian ghini, Romanian bine, Sardinian beni.

The surprising sound shift from Old Latin du- to Latin b- is well documented: duonus "good" > bonus; duellum "war" > bellum.