Okay, BH, you're almost there. Below image and text taken from the National Museum of the United States Air Force,
http://www.wpafb.af.mil/museum/index.htm
I would submit a more authoritative source than wikipedia, and still no sign of the extravagant mods in sasless's post.
The XB-40 was developed to test the bomber escort concept. Early in World War II, before long range fighters like the
P-51D became available, the Air Corps developed plans to modify bombers with additional defensive armament. These aircraft would accompany regular bomber formations and provide protection from attacking fighters. Unfortunately, the modified escort bombers were not maneuverable enough to protect large formations. They were also excessively tail heavy and overweight. A standard
B-17F formation returning from a mission (after bomb release) outpaced the escort bombers. The YB-40 participated in only a few operational missions in mid-1943 before being withdrawn from service.
The prototype XB-40 was modified by Lockheed Vega (Project V-139) by converting the second production B-17F-1-BO (S/N 41-24341). Defensive armament consisted of fourteen .50-cal. machine guns in seven gun positions: chin turret, two dorsal turrets, ball turret, two waist positions and the tail. The first flight of the XB-40 was on 10 November 1942. Twenty service test aircraft were ordered (Vega Project V-140) as YB-40 along with four crew trainers designated TB-40. Because Vega had higher priority production projects, the YB-40/TB-40 assembly job was transferred to Douglas.
In the spring of 1943, the 327th Bomb Squadron of the 92nd Bomb Group, RAF Alconbury was assigned twelve YB-40s for operational combat tests. The first mission was flown on 29 May 1943 and after less than ten missions the aircraft were withdrawn from service. Most were converted back to the standard B-17F configuration.