point of order , it was the AIR CREW that was limited to 25 missions, the AIRPLANES had no mission limit, they flew until they could not fly any more.
the crew of the Mamphis Belle was the 1st to finish their 25th mission and were sent back to the states, the B17 Mamphis Belle flew one more mission before the army decided to send it back to the states for a war bond promo.