So, I bought a few more vintage books over the holidays, and one of them - a children's book - definitely reinforces the connection with battleships. A few selections from the text:
James F. Wallace wrote:
The "Coronado" is a flying battleship. It is the pride of the U.S. Navy's bombing fleets.
James F. Wallace wrote:
The Martin "Mariner" is another air battleship of the U.S. Navy, for mid-ocean patrol.
James F. Wallace wrote:
Patrol bombers, or flying boats, are air battleships. They roam far out to sea, hunting enemy ships and submarines. They are as big and deadly as the Army's heavy bombers.
(Source: James F. Wallace,
War Planes in Action (Racine, Wisconsin: Whitman Publishing Company, 1942), [unnumbered pages].)
While none of the above refer to the B-17, the sheer number of times the metaphor is used demonstrate that the idea was deeply ingrained in the public consciousness. Furthermore, the following passage, while not mentioning the B-17 specifically,
is a description of land-based "mediums" and "heavies":
James F. Wallace wrote:
The crews include as many as 10 men-the pilot (who is the ship's captain), the co-pilot, the bombardier, radio man, gunners, and the navigator (who works out the ship's course for flights up to 4000 miles, just as if he were planning the course of a ship at sea). Each heavy bomber bristles with guns.
(Source: James F. Wallace,
War Planes in Action (Racine, Wisconsin: Whitman Publishing Company, 1942), [unnumbered pages].)
The fact that "ship" was already a common term for any type of airplane at the time would have only meant that the connection to vehicles of the water-based variety was that much easier to make.
Finally, just to further expand the preponderance of applications of the "Flying Fortress" moniker at the time, here's a caption from another book:
David C. Cooke wrote:
The Fascist Flying Fortress. From this front view, the Piaggio P. 108C heavy bomber appears almost identical with the U.S. Boeing B-17.
(Source: David C. Cooke,
War Planes of the Axis (New York: Robert M. McBride & Company, 1942), 171.)