kingshmegland is correct; U2 has almost always been pretty daring and not afraid to experiment with their sound. The irony is that a lot of people think they always sound the same, but I think that has mostly to do with Bono's voice and Edge's guitar sound being so identifiable that when some hear either or both, their reaction is, "Yep, sounds like U2." But think of how ballsy is was to do what they did with Achtung Baby, radically changing their sound like that when they were at the time already the biggest (regular working*) band in the world. That could have backfired on them, but it did the opposite: it solidified their standing as the biggest band in the world at the time (which was no small feat, considering how big Metallica and Guns N' Roses were in the early 90s).
*regular working meaning bands that regularly still recorded and toured, as opposed to bands like the Stones, Floyd, The Who, etc.