I remember the B-17 in Civ V, but there was an earlier Civ or Civ-variant that used the Lancaster. Now I'm going to go quite mad until I track down which game it was!
IF America is going to get a Strategic Bomber UU, and I agree on its visibility if not its actual effectiveness in cost to damage inflicted ratio, then the game also has to get strategic bombing, and the air war in general, Right. Fighter interception, the extreme cost of building, manning, and developing the unique technical skills to make strategic bombing even marginally effective (formation flying, navigational aids, bomb sights, specialized bombs and munitions, escort flying and the aircraft to do decent escort work, etc.), close air support as the alternative to the strategic war, the peculiarities of naval air forces and naval aircraft, etc. Note that only two countries in the world ever developed real strategic air forces, and then doing so took an enormous percentage of their manpower and industrial capacity - nobody else even came close. Germany and the USSR's share of 'heavy bombers' (4-engined, long range, 4 tons or better bomb load) were measured in the dozens compared to the USAF and RAF forces measured in the thousands.
Getting the Wild Blue Yonder right in the game would also open up some other potential UUs, like the German Ju-87 Stuka or Soviet IL-2 Sturmovik. Right now, in Civ VI, no air UU is of much use since the AI is apparently oblivious to all air units so that 'air combat' simply does not happen - I've never seen an air battle in over 3000 hours of playing time in Civ VI, which is pretty pathetic.