Battle of the Atlantic was not highest priority for many, B17 was useful but the VLR Lib was a key part in the final victory for the Allies, 17+ hours of flying time was exceptional and probably did more to close the Atlantic Gap than any other aeroplane.
Libs over occupied Europe were an inflammable liability with limited bomb loads. The wonders of hindsight indicate that the daytime raids should have been undertaken by Mossies accompanied by whatever escort fighters the Americans were using. The Mossies could always stooge around for a bit if the Yanks had difficulty keeping up...