Murder in the Clouds (1934)

Bob Halsey is a first-rate pilot who's in love with stewardess Judy Wagner. He's ordered to deliver a secret formula to Washington, D.C., but a spy hears about the assignment and sabotages it by murdering Bob's fellow flyers and making off with the liquid. While the government conducts a vast search for the formula, the spies entangle Judy in their web of deceit, causing Bob to set off on his own in an effort to save his sweetheart and retrieve the missing mixture.