- David O. Selznick's experience of trying to have life-saving serum flown in for his critically ill brother was the basis for the flying sequences ending the movie.
- Special effects technician Edmund E. Fellegi was killed when he fell from a 40-foot catwalk while releasing balloons for the New Year's Eve party scenes.
- Seven actors listed in records for this movie were not seen in the final print. These are (with their character names) Jack Mulhall (Rock Springs Radio Operator), Arthur Gardner (Iowa City Radio Operator), John M. Sullivan (John Hopkins Chemist), John Austin (Allentown Radio Operator), and the three Doctors, Robert Strange, Perry Ivins and Gladden James.
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