Yup, but that's not what you stated. The O/100 was the grandfather of the strategic bomber. It was used by the RNAS for strategic bombing attacks, as was its successor the HP O/400 by the RAF, and then there was the big V/1500, which was designed specifically to fly to Berlin and back. Surely these qualify as the grandfather of strategic bombers well before the XB-19, especially since the former two were actually used in the role in combat.