They were given the "fortress" moniker because of the concept in the 1920s and 30s that heavy bombers were all that were needed to defend a country or project power. Fixed fortifications were unnecessary, as were armies and navies. Even fighters were thought of a obsolete, as "bombers would always get through".
The name was actually given by a news reporter seeing the prototype b17 or model 299. He said with all those guns it was a flying fortress. Boeing like the name and used it.
This was the bomber nick named the Flying Fortress right?