What was the Boeing YB-40 Flying Fortress and why was it built?

The Boeing YB-40 Flying Fortress was a heavily armed B-17F service-test aircraft built during the Second World War to escort US daylight bomber formations beyond fighter range. It gave up the normal bomb load for extra .50-calibre guns, turrets, ammunition and armour, but proved too heavy to keep pace after the formation dropped its bombs.

For Aviation & Real-World Flying readers, the key distinction is that the YB-40 was not a clean-sheet bomber or an oversized escort fighter. It was a B-17F converted into an escort gunship intended to remain within a bomber formation and reinforce its defensive fire.

Why was the YB-40 Flying Fortress built?

The YB-40 was built to protect American heavy bombers during the part of a mission that contemporary Allied fighters could not reach. In 1942 and early 1943, B-17 formations attacking occupied Europe and Germany often had fighter cover near England but faced deep portions of the route without it.

US daylight bombing doctrine depended on compact formations whose overlapping machine-gun fields would discourage fighter attacks. In practice, damaged aircraft, gaps in the formation and concentrated head-on attacks still created vulnerable points. Our account of B-17F daylight bombing and defensive armament explains the wider operational problem.

The proposed answer was to place heavily armed B-17s among the bombers. Each YB-40 would carry more guns, ammunition and armour than a normal B-17F, concentrating fire against Luftwaffe fighters while the other aircraft carried the bombs.

Was the YB-40 a gunship?

Yes, the YB-40 was a gunship in the broad sense, but it was specifically an air-to-air escort gunship. Its guns occupied conventional nose, dorsal, ventral, waist and tail positions and were intended to engage attacking aircraft.

It should not be confused with later gunships designed to circle ground targets and fire side-mounted cannon. Nor was it an escort fighter: the YB-40 lacked the speed, climb and manoeuvrability to intercept or pursue enemy fighters away from the bomber stream.

How did the YB-40 differ from a standard B-17F?

The YB-40 retained the B-17F airframe and engines but traded useful load and performance for much heavier defensive armament. A flyable standard B-17F comparison for FS2004 provides a useful visual baseline for the underlying aircraft.

Published armament totals commonly range from about 14 to 16 .50-calibre machine guns. The figure varies because the XB-40 prototype, service-test aircraft and later experimental fits were not identical, while flexible nose and cheek positions are not counted consistently. The YB-40 should therefore be identified by its overall configuration rather than one quoted gun total.

How can you recognise a YB-40 plane?

The clearest recognition feature is the second powered dorsal turret behind the normal upper turret. Other clues include the powered chin turret and unusually dense waist armament.

• Check the upper fuselage: a genuine YB-40 configuration has two dorsal turrets, not one.
• Look beneath the nose: the powered chin turret distinguishes it from most ordinary B-17Fs.
• Inspect the waist: service aircraft could carry paired guns rather than the usual single gun at each side position.

A chin turret by itself does not identify a YB-40. That feature appeared on late or modified B-17Fs and became standard on the B-17G; our FSX B-17G example shows the characteristic G-model nose without the YB-40's second dorsal turret.

This distinction also matters when choosing a simulator model. A texture-only repaint cannot add the second turret, altered waist stations or correct gunship silhouette. A B-17F or B-17G wearing YB-40 markings is only an approximation unless the add-on supplies dedicated external geometry.

Did the YB-40 work in combat?

The YB-40 fought in combat, but it did not meet the escort requirement well enough to enter production. Service-test aircraft operated from England with the 92nd Bomb Group, principally through the 327th Bomb Squadron, during May, June and early July 1943.

The crews could add substantial fire to a formation and received claims for enemy fighters. The concept nevertheless suffered from problems that extra guns could not solve:

• Weight and drag: turrets, armour, ammunition and associated equipment were added without a corresponding increase in power.
• Post-target performance: ordinary B-17s accelerated and climbed better after dropping their bombs, while the YB-40 had no comparable reduction in weight.
• Limited tactical reach: the gunship could reinforce nearby bombers but could not rapidly cross the bomber stream or chase an attacker.
• Reduced bombing efficiency: an aircraft and crew committed to the YB-40 role contributed little or no normal bomb load.
• Maintenance burden: additional powered turrets, ammunition feeds and gun positions created more equipment to service.

The formation mismatch was decisive. The YB-40 could remain with bomb-laden B-17s on the outward leg, yet risked falling behind during the vulnerable withdrawal from the target. Better drop-tank arrangements and longer-ranged fighters, particularly the P-38 and P-51, offered real escort performance without sacrificing bomber payload.

What does YB-40 mean, and was there a B-40 bomber?

The official service-test designation was YB-40; no standard production B-40 bomber followed it. The shortened forms “B40 bomber” and “B-40 Flying Fortress” normally refer to the YB-40, but they omit its significant test-status prefix.

• X in XB-40 identified the experimental prototype stage.
• Y in YB-40 identified aircraft undergoing service testing.
• B placed the design in the bomber category; it did not stand for Boeing.
• 40 was the allocated bomber design number, not a reference to weapon calibre.

The project is usually summarised as one XB-40 prototype, 20 YB-40 service-test conversions and four related TB-40 trainers, giving 25 project aircraft. The smaller figure of 13 found in some accounts generally describes the principal batch associated with the European operational trial rather than the complete programme.

Was the Boeing YB-40 actually built by Boeing?

Calling it the Boeing YB-40 is accepted because it belonged to the Boeing-designed B-17 family, but the attribution needs qualification. Vega Aircraft Corporation, a Lockheed subsidiary and B-17 production partner, undertook the original XB-40 conversion.

Boeing, Douglas and Vega all participated in wartime Flying Fortress production, with additional modification work passing through the broader B-17 manufacturing system. The YB-40 was therefore a modified Boeing design, not an entirely new aircraft created and manufactured solely by Boeing.

What did the YB-40 contribute to the B-17G?

The escort-gunship concept was abandoned, but the YB-40 helped demonstrate the value of a powered chin turret against frontal attacks. Stronger forward defence subsequently became one of the defining features of the production B-17G.

The YB-40 did not simply evolve into the B-17G. The G remained a bomb-carrying Flying Fortress with one dorsal turret and a much less extreme defensive fit; the second dorsal turret, paired waist arrangement and exceptional ammunition load of the YB-40 were not adopted as its standard configuration.

The experiment's lasting lesson was clear: additional forward firepower could improve the B-17, but a heavily burdened bomber could not replace a true long-range escort fighter.