Here is the answer to my aviation quiz.
All four airplanes have the main landing gear wheels exposed when retracted. In other words, there are no doors covering the main landing gear wheels, as shown below.
While this was common in the 30s, it is very uncommon today. As far as I know, the Boeing 737 is the only common modern airliner with exposed wheels in flight. I have pointed this out to people who are universally surprised to see the exposed wheels on a 737 climbing out from a takeoff.