From a web page by Greg Krenzelok

The navigator was the master of the instruments. He used his good sense of direction and dead reckoning in tandem with the airspeed and elapsed time to calculate approximate position. The use of pilotage, or the sighting of ground landmarks was fine for daylight runs with good weather, yet it afforded the enemy an equal advantage. Therefore, the navigator was well versed in celestial navigation and instrumentation. My father claims that all combat navigation was by pilotage in case of a necessary break from formation due to damage and the necessity for visual contact with the target. I have found that most textbooks agree. In fact, " The Soldier: B-24 Liberator," claims that many lead or formation ships would substitute a navigator for the nose gunner, "just to watch the ground." The increased use of navigational instruments meant a corresponding increase in calibration. The most important function of the navigator was to be certain the instruments were calibrated. In fact, the best navigator in the war may be useless during a night bombing raid, or during flak bursts, or in poor weather. Thus, he was responsible for maintaining an alternate route back to base in the event of separation from the formation for whatever the reason. His knowledge was invaluable and the key to the success was the dialogue between himself and the pilot. The discussion concerned weather, alternatives, course, and airspeed, with any changes being discussed between both crewmen.