A gondola was a ventral casemate-style structure used on many World War II-era military bomber aircraft, especially on German designs, where they were usually known as Bodenlafette, often shortened to Bola (from German Boden, 'floor', + Lafette 'gun carriage or mounting', from French l'affût, gun carriage).
Gondolas housed either a gunner or a bombardier.
Examples of gondolas on World War II military aircraft:
Other types of aircraft equipped with gondolas: