The Boeing B-9 was the first all-metal monoplane bomber.  The Model 215 made its inaugural flight on April 29, 1931.  The YB-9 was dubbed the "Death Angel" and was praised as the World's Fastest Bomber.  It had a radical design with its low monoplane wing and semi-retractable landing gear, with wheels partly exposed.  It carried a crew of five with four seated in open cockpits.  The radio operator sat forward and below the pilot.  Although only seven aircraft were produced, the B-9 had many innovations that would be utilized on the B-17. 

After Boeing failed to receive a contract to produce the B-9, a design team began work on the model 299 prototype in June 1934.  The first flight of the Model 299 was on July 28, 1935.  The Model 299 was redesignated the B-17A.  Early B-17s still had the shark tail fin similar to the B-9 and it wasn't until the B-17E models that the tail was enlarged with a huge dorsal fin creating the classic look of the B-17 commonly recognized today.  The B-17 served in almost every theater of World War II, but is most famous for use by the Eighth Air Force, based in the UK, to bombard German targets with daring daylight missions.  Its heavy armament and sturdy construction justified the name -                 

                                                  "Flying Fortress".