Boeing YB - 40 Flying Fortress The Boeing YB - 40 Flying Fortress was a modification of the United States B - 17 Flying Fortress bomber aircraft , converted to act as a heavily armed " gunship " for other bombers during World War II . At the time of its development , long - range fighter aircraft such as the North American P - 51 Mustang were just entering quantity production , and thus were not yet available to accompany bombers all the way from England to Germany and back . Design and development . Work on the prototype , Project V - 139 , began in September , 1942 by converting the second production B - 17F - 1 - BO ( serial number 41 - 24341 ) built . Conversion work was done by Lockheed ' s Vega company . The aircraft differed from the standard B - 17 in that a second manned dorsal turret was installed in the former radio compartment , just behind the bomb bay and forward of the ventral ball turret ' s location . The single . 50 - calibre light - barrel ( 12 . 7 mm ) Browning machine gun at each waist station was replaced by two of them mounted side - by - side as a twin - mount emplacement , with a mount for each pair of these being very much like the tail gun setup in general appearance . The bombardier ' s equipment was also replaced with two . 50 - calibre light - barrel Browning AN / M2 machine guns in a remotely operated Bendix designed " chin " - location turret . The existing " cheek " machine guns ( on the sides of the forward fuselage at the bombardier station ) , initially removed from the configuration , were restored in England to provide a total of sixteen guns , and the bomb bay was converted to an ammunition magazine . Additional armor plating was installed to protect crew positions . The aircraft ' s gross weight was some 4 , 000 pounds greater than a fully armed B - 17 . An indication of the burden this placed on the YB - 40 is that while the B - 17F on which it was based was rated to climb to 20 , 000 ft in 25 minutes , the YB - 40 was rated at 48 minutes . Part of the decreased performance was due to the weight increase , and part was due to the greater aerodynamic drag of the gun stations . The first flight of the XB - 40 was on 10 November 1942 . The first order of 13 YB - 40s was made in October 1942 . A follow - up order for 12 more was made in January 1943 . The modifications were performed by Douglas Aircraft at their Tulsa , Oklahoma center , and the first aircraft were completed by the end of March 1943 . 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 . A variety of different armament configurations was tried . Some YB - 40s were fitted with four - gun nose and tail turrets . Some carried cannon of up to 40 mm in calibre , and a few carried up to as many as 30 guns of various calibers in multiple hand - held positions in the waist as well as in additional power turrets above and below the fuselage . Externally , the XB - 40 had the symmetrical waist windows of the standard B - 17F and the second dorsal turret integrated into a dorsal fairing . In contrast , most of the YB - 40s had staggered waist windows for better freedom of movement of the waist gunners , and the aft dorsal turret was moved slightly backwards , so that it stood clear of the dorsal fairing . Operational history . The YB - 40 ' s mission was to provide a heavily gunned escort capable of accompanying bombers all the way to the target and back . Of the initial order of 13 , one ( 43 - 5732 ) was lost on the delivery flight from Iceland to UK in May 1943 ; it force - landed in a peat bog on an offshore Scottish island after running out of fuel . Although removed to Stornoway and repaired , it never flew in combat . The remaining 12 were allocated to the 92d Bombardment Group ( Heavy ) , being assigned to the 327th Bombardment Squadron , stationed at RAF Alconbury ( AAF - 102 ) on 8 May 1943 . YB - 40s flew in the following operational missions : Summary . Altogether of the 59 aircraft dispatched , 48 sortie s were credited . Five German fighter kills and two probables ( likely kills ) were claimed , and one YB - 40 was lost , shot down on the 22 June mission to Hüls , Germany . Tactics were revised on the final five missions by placing a pair of YB - 40s in the lead element of the strike to protect the mission commander . Overall the concept proved a failure because the YB - 40 could not keep up with standard B - 17Fs , particularly after they had dropped their bombs . Despite the failure of the project as an operational aircraft , it led directly to modifications conspicuous on the final production variant of the B - 17 , the B - 17G : Once the test program ended , most of the surviving aircraft returned to the United States in November 1943 and were used as trainers . 42 - 5736 ( " Tampa Tornado " ) was flown to RAF Kimbolton on 2 October 1943 where it was put on display and later used as a group transport . It was returned to the United States on 28 March 1944 . All of the aircraft were sent to reclamation , mostly at RFC Ontario in May 1945 , being broken up and smelted . ( A couple of the YB - 40s can be seen in the 1946 movie " The Best Years of Our Lives " , in the famous scene shot at the Ontario " graveyard " . ) No airframes were sold on the civil market .