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The YB-40 was an attempt to provide a long range escort aircraft to support the Eighth Air Force’s daylight bombing campaign over Europe. It was created by adding extra guns to a standard B-17F, bringing the total of 0.50in guns carried up to fourteen and by carrying a massive amount of ammunition, three times as much as on the normal aircraft.
Every gun position on the aircraft apart from the existing upper turret was upgraded in some way on the YB-40. It was the first version of the B-17 to feature the chin turret that would become a standard feature on the B-17G. The rear firing guns in the radio operator’s compartment were replaced by a new powered turret. The single waist guns were replaced by dual mountings, and were given hydraulically boosted controls, as were the twin tail guns.
Early work on developing the YB-40 began on 25 June 1941. The idea was the subject of a conference held in the Office of the Director of Military Requirements in Washington on 1 April 1942, and the idea was approved eight days later. On 10 May Vega aircraft were given the contract to develop the escort aircraft. At the same time a separate project was under way to install a Bendix chin turret on a B-17F. In July Vega were invited to examine this project, and on 2 August 1942 the test aircraft was delivered to them, complete with a mock-up of the chin turret and a power-operated tail gun. This became the XB-40 prototype, and was delivered to the USAAC for testing on 19 November 1942. Initial tests were satisfactory. Vega were given the contract for the engineering work, while Douglas Aircraft received a contract to convert the aircraft at the Tulsa Modification Facility. On 7 October 1942 a firm contract was put in place for six conversions, later increased to thirteen. Douglas submitted their quote, for $469,820, on 2 January 1943. A lower price was agreed, and work proceeded at speed.
In April 1943 the first YB-40s were sent to Orlando, where they took part in tests to see if they could work with the standard B-17F. These tests revealed no major problems, and the aircraft were flown to Britain. They flew their first mission on 29 May 1943, escorting a raid against St. Nazaire. Eight aircraft were allocated to the raid, although only seven were able to take part. Two acted as wing aircraft for the lead aircraft of the bombardment groups involved, while the remaining aircraft formed the low squadron of the lead group, normally a vulnerable position. Unfortunately the Germans failed to cooperate with the test, and only one of the YB-40s was actually attacked. This attack was aborted after the chin turret opened fire, beginning to prove the validity of that part of the concept. The chin turret would become a standard feature on the B-17G.
Only one XB-40 was lost to German attack, during a raid over the Ruhr on 22 June. Nevertheless, on 7 July 1943 the commanding officer of the 92nd Bombardment Group submitted a report that listed numerous problems with the guns and recommended that no more YB-40s should be sent to the European theatre.
The YB-40 concept suffered from two major problems. The first was that the heavily laden gun ships couldn’t keep up with the standard bombers after they had dropped their bombs. The second was that a single YB-40 in a formation didn’t actually provide that much extra firepower. The B-17F carried eleven 0.50in guns, while the B-17G could carry as many as thirteen. It was calculated that they were only 10% more effective at repelling German fighters than the standard B-17F, most of which was due to the chin turret. The program lingered on until September 1943 when it was finally cancelled, after 22 YB-40s had been produced.