The Douglas B-23 Dragon was the Army Air Corps first bomber with a manned tail gun emplacement featuring a .50-caliber gun and was introduced in 1939 a successor to the Douglas B-18 Bolo. (seen here at VG: http://virtualglobetrotting.com/map/douglas-b-18-bolo-2
While faster and better armed than the B-18 Bolo, the B-23 was not comparable to newer medium bombers like the North American B-25 Mitchell and Martin B-26 Marauder. For this reason, the 38 B-23s built were never used in combat overseas, and many were converted to transports as the UC-67. Howard Hughes used converted B-23s as personal aircraft.