The YB-40 was a derivation of the Boeing B-17, in which additional guns were added in hopes that these heavily-armed B-17s would be able to "escort" the other bombers, protecting them from enemy fighter attacks when beyond the range of friendly fighter escort. Changes included a remotely-controlled chin turret (which later became standard on the B-17G model), twin .50 caliber guns in the waist positions (instead of the single guns), an extra twin gun power turret behind what used to be the radio operator's gun position, and sometimes extra cheek guns in the nose. The bombs and bombardier were left behind, and extra ammo for the guns was carried. Even with the reduction in weight without the bombs, the added weight of guns and ammo made the aircraft heavier and slower. The program was made obsolete not only by its own failures, but by the appearance of long-range fighter escorts that could fill the role much more ably. Some photos of YB-40 can be found in the link below:
http://aviacaoemfloripa.blogspot.com.br/2011/02/boeing-yb-40-um-gunship-na-segunda.html
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