Well, the Lockheed Hudson, was, after all, an airliner. They were available early in the war and in fact were used to pick up and drop off agents in France. They had two .303 machine guns mounted forward of the cockpit. I recall reading of one such mission in which they received reports that Luftwaffe nightfighters were around. So the pilots decided to see if the machine guns actually worked. That test had the effect of scaring the bejesus out of the passengers.
Hudsons were also used for the "Scrutanator" missions to Switzerland and Sweden, before being replaced by BOAC Mossies when it was found they needed a bit more poke.