The second production B-17F-1 was converted into the first YB-40 Prototype. The development of the Lockheed – Vega built XB-YB-40 ,was the Army Air Forces attempt to bring additional defensive firepower to the bomber formations flying against the German Luftwaffe fighters.The X in XB was used for” EXPERIMENTAL” and the Y in YB signified ” SERVICE TEST “.They were rushed into combat in May, 1943 with the 92ndBG at ALCONBURY.
These aircraft were to fly on the wings of the regular formations and provide extra FIRE power. Twenty two aircraft were built , one -XB-40 and twenty two YB- 40s. It carried a crew of nine , without a bombardier. The aircraft probably should have been given a “P ” for pursuit designation because it was more like a fighter than a bomber. No bomb bay , bombs or bombardier.
The YB-40 was crammed with ammunition . They carried triple supplies , 11,275 rounds compared to 3,900 rounds carried on a standard B-17F. The aircraft was modified with an additional BENDIX turret mounted in the place of the radio compartment guns. Additionally a TWIN CHIN turret was mounted and the SINGLE. waist gun was replaced by TWIN GUN mounts . Both waist guns and TWIN GUNS were hydraulically boosted for better control. Receptacles for increased ammunition were provided for all positions.
The YB40s retained full bombing capabilities although seldom used , due to the extreme weight of the extra ammunition load. Their career came to a quick end when it was discovered that the heavier YB-40s could not keep pace with the mainbomber stream they were intended to protect . Since no one wanted to be a straggler in the campaigns against the LUFTWAFFE the YB-40 s were now more a burden than a help. Although this aircraft provided massive firepower, the two additional gun positions on each ship did not add materially to the combined firepower of a Group formation. By Aug, 1943 all the YB-40s were withdrawn from combat and the YB-40 Program was discontinued.