A few years ago we had a B-17 spend a few days out our local airport and it did some runs above town. It was several hundred feet up (probably even over 1000ft) and still quite loud, but my favourite memory has to be heading to work morning, sitting at an intersection and seeing it flying low (maybe 100 or 200 feet) over the farmland parallel to the highway before banking left and following the road back to the airport. Quite impressive.