The Boeing B-17 Flying Fortress is a four-
engined heavy bomber developed in the
1930s for the United States Army Air
Corps (USAAC). Competing against
Douglas and Martin for a contract to build
200 bombers, the Boeing entry (prototype
Model 299/XB-17) outperformed both
competitors and exceeded the air corps'
performance specifications. Although Boeing lost the contract (to the Douglas B-18 Bolo) because the prototype crashed,
the air corps ordered 13 more B-17s for
further evaluation. From its introduction in
1938, the B-17 Flying Fortress evolved
through numerous design advances,

becoming the third-most produced
bomber of all time, behind the four-
engined B-24 and the multirole, twin-
engined Ju 88.

B-17G

By the time the definitive B-17G appeared,
the number of guns had been increased
from seven to 13, the designs of the gun
stations were finalized, and other
adjustments were completed. The B-17G
was the final version of the Flying Fortress,incorporating all changes made to its
predecessor, the B-17F, and in total,
8,680 were built, the last (by Lockheed)
on 28 July 1945. Many B-17Gs were
converted for other missions such as
cargo hauling, engine testing, and reconnaissance.
Initially designated SB-17G, a number of B-17Gs were also
converted for search-and-rescue duties,
later to be redesignated B-17H.

Wikipediea

Controls

AG1: drop all payload

AG2: chin and rear turret

AG3: dorsal and ball turret