Prior to the mass availability of long-range fighter escorts for its bomber formations, the USAAF undertook serious trials with various bomber airframes converted as flying gunships. The concept involved a heavily-armed and -armored aircraft accompanying the bomber fleet into enemy airspace, supplying point defense against intercepting fighters, and guiding the formation back home under protection. Such conversion work involved several of the classic American bombers of the war including the Boeing B-17 "Flying Fortress" and the Consolidated B-24 "Liberator". The former was represented by the "YB-40" (detailed elsewhere on this site) and the latter by the "XB-41" detailed below.
A B-24D production model was set aside for the conversion work by Consolidated and this involved installation of additional guns and armor protection, the bombing equipment being deleted as a result. A second twin-gunned dorsal turret was added aft of the first and a remote-controlled Bendix powered turret was as added to the "chin" position. Each single-gunned beam position now showcased a twin-gunned arrangement. Two more guns were located at the tail and the Sperry ball turret, with its two machine guns, was retained at its ventral position. Due to the Liberator's low ground profile, the Sperry turret was retractable on take-off and landing actions. To feed the multiple machine gun arrangement, some 12,420 rounds of 0.50 caliber ammunition was to be taken aloft - as such, the bomb bay was used as a reserve for thousands of rounds.
Work on the XB-41 began in 1942 and the sole prototype was handed over for testing in January of 1943. After just two months of evaluation, the aircraft was written off from contention and this cancelled an initial batch of thirteen examples. It was found that the aircraft gained too much weight and added drag from the added armament, ammunition stocks, and armoring. The exposed positions also added considerable drag making the flying Gun Bus much slower than their intended bomber formations - particularly when the formations had dropped their bomb loads. More work on done on the prototype to make it a viable escort platform but this led to little improvement and the attempt ultimately fell to aviation history.
The prototype was reworked as a trainer and redesignated as "TB-24D". It ended its days in this form until scrapped during February of 1945.
As completed, the XB-41 was crewed by nine personnel. Overall length was 66.3 feet with a wingspan of 110 feet and height of 17.10 feet. Maximum Take-Off Weight (MTOW) was 63,000lb. Power was from 4 x Pratt & Whitney R-1830-43 air-cooled radial piston engines delivering 1,250 horsepower each. Maximum speed was 289 mph with a range out to 3,100 miles and a service ceiling of 28,500 feet.
The YB-40 (B-17F) Gun Bus project fared slightly better as twenty-five of the type were produced though only in its developmental form. This project was also largely viewed as a failure for the return of investment though it brought about key qualities featured in future B-17 operational models - namely the Bendix powered chin turret featured in the B-17G.
All of this was moot as the USAAF began receiving long-range fighter escorts, complete with jettisonable fuel tanks, in number and these served well in providing protection to bomber formations over enemy territory. They were nimble aircraft with excellent performance and good armament capable of flying with the formations to-and-from the target areas.