YB-40
From Wikipedia:
The Boeing YB-40 Flying Fortress was a modification of the B-17 Flying Fortress bomber aircraft, converted to act as a heavily-armed escort for other bombers during World War II. At the time of its development, long-range fighter aircraft such as the P-51 Mustang were not yet available to accompany bombers all the way from England to Germany and back.
The aircraft differed from the standard B-17 in that a second dorsal turret was installed between the top turret and the waist guns; and the single 0.50-calibre (12.7 mm) machine gun at each waist station was replaced by a pair of 0.5-calibre (12.7 mm) guns. In addition, the bombardier's equipment was replaced with two 0.50-calibre (12.7 mm) machine guns in a 'chin' turret to augment the existing 'cheek' machine guns, and the bomb bay itself was converted to a magazine.
The concept was twofold. First, the YB-40 would provide a heavily-gunned escort capable of accompanying the bombers all the way to the target and back. Second, they were used as decoys; a YB-40 would leave the bomber stream with one engine feathered, apparently in distress. Enemy fighters would close for the kill and discover that the 'cripple' was nothing of the kind. The aircraft was used with some success in the Mediterranean Theater of Operations, but was overall a failure because it could not keep up with standard B-17Fs once they had dropped bombs. It was withdrawn from service after fewer than ten missions. Some 25 were built. Operational units were stationed with the 92nd Bomb Group (H) at their base in Poddington, England.
The first mission flown by a YB-40 took place on May 29 1943, against Saint Nazaire. In the nine missions flown by the planes, five kills and two likely kills were claimed, and one YB-40 was lost. Despite the failure of the project as an operational aircraft, it did provide insight that led to the addition of a chin turret on the B-17G variant of the original Flying Fortress.