Clouds Over Europe (1939)

Experimental aircraft of several countries are mysteriously vanishing during test flights over the sea. The latest is British, and Hammond of the Secret Service, comical but competent, investigates undercover. Also interested is Hammond's sister Kay, a reporter working under cover as a waitress. The government and plane manufacturer Barrett think the whole business is just coincidence, but the "Viking", ostensible salvage ship controlled by a foreign power, has much to do with it.