The RAF used the Fortress for high altitude DAYLIGHT bombing and it was a dismal failure
Tactics were part of the problem, the first raid was by three aircraft and those that followed were mostly by individual aircraft, problems were had with guns freezing at altitude, issues with the Sperry bomb sight, no rear defensive armament. Between the aircrafts initiation 8 July and removal from the bombing role 26 September they flew 26 raids, 51 sorties, of which 25 had been aborted with no bombs dropped. Of the 50 tons of bombs dropped only one ton is estimated to have hit the target, eight aircraft lost - one undercarriage collapse on landing (aircraft cannibilised for spares), one aircraft disintergrating on touch down due combat damage, one accidentally burnt on the ground, three destroyed by fighters on Norway raid, one lost on high altitude test, one appeared in a vertical dive from cloud into the ground. Aborts due aircraft unservicabilities seem to back up the USA advice that the aircraft was not yet ready for service.