The movie Q Planes got it’s title post-production. It is assumed the name was derived from the Q-Ships that were popular in World War I.
These were freighters or steamer designed to look like non-military ships that would be easy targets for German U-boats. The idea was to get the U-boat to surface, and then the Q-ship would reveal its guns and blast away. In the movie Q Planes, no one ever does this with one of the planes. Nor is the term Q Planes ever used in the film! The title appears to have been thought of more for marketing to UK audiences than for how it relates to the plot.
The British public loved Q-ships in World War I, but they were not terribly effective according to some historians, and even less effective in World War Two. The designation “Q” came from the ships being outfitted in Queensland, Ireland, and it helped promote the idea of “Q” being a designation for taking something ordinary and outfitting it to be a deadly weapon.
This clip lets you see a real Q-Ship. It’s old footage showing the troops and a Q-ship in action.