The back-up possibility to use a Lancaster is mentioned in General Grove's account of the Manhattan Project
"Now it can be told" A great read from a management perspective with all the various challenges and obstacles that had to be overcome. Should be required reading at Staff College in my opinion. General Groves was selected for the project due to his excellent reputation and recent work overseeing the building of the Pentagon (as a Major). He did get promoted to Lt Col for the Manhattan Project and later on to General during the project.