Trivia: The crewmembers of the Memphis Belle are not based on actual crewmembers. They are fictitious characters.
Trivia: Very few flyable examples of the B-17 existed at the time of the shooting of the movie. One airplane "stood in" for several by having its decals changed. The B-17G featured in the film has since undergone a meticulous restoration and now lives in Renton, Washington, USA. Though it is fully flyable, certain certification issues with the Federal Aviation Administration have kept it grounded.
Answer: That would be a flare being fired from the aircraft. Yellow ones meant the aircraft had sustained serious damage, although that was often self-evident, and a red one meant that there was a seriously injured crewman on board - prompting first aid teams to be ready to meet the aircraft once it had landed. Their was also an unofficial green flare that meant someone had completed their tour of duty.
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