At the end of Chapter IV, when Gen. Savage's plane is taxiing to the hardstand after the mission, the two inboard engines are off, the two outboard engines are running. The next scene pans from Gen. Crowe and Maj. Stovall, then to the side of the aircraft, and back to the front of the bomber. At that point the right inboard engine is now running and the outboard engine is off.
In the Epilogue Savage is writing a letter to Temple's widow the date is June 15, 1942. Eighth Bomber Command, later 8th Air Force, didn't start B-17 operations until August 17, 1942 when 12 B-17s bombed Rouen, France. The date of letter and the mission should have been late 1942 at the earliest.
In the opening of the Epilog segment, Frank is writing a letter to Major Temple's wife and the date at the top of the letter is June 15th, 1942. Since General Savage is a career military officer he would have more likely used the military dating convention of 15 June 1942 instead.