Learn what the Boeing YB-40 Flying Fortress was, why the B-17 escort gunship was built, how it differed and why combat trials failed.
The Boeing YB-40 Flying Fortress was a heavily armed escort version of the B-17F, created for the US Army Air Forces in the Second World War. It was built to protect daylight bomber formations beyond fighter range, using extra turrets, guns, ammunition and armour instead of carrying a normal bomb load.
For readers exploring Aviation & Real-World Flying history, the crucial point is that this was not a clean-sheet aircraft. It was a B-17F modified into an escort gunship, intended to remain inside the bomber formation rather than chase enemy fighters like a conventional escort.
Why was the YB-40 built?
The YB-40 was built because Allied fighters initially lacked the range to escort heavy bombers throughout deep raids over occupied Europe and Germany. During 1942 and early 1943, B-17 formations could therefore spend part of a mission without fighter protection.
US daylight bombing doctrine relied on tightly grouped bombers combining their defensive fire. Battle damage, formation gaps and concentrated fighter attacks could still leave individual aircraft exposed, so planners proposed mixing heavily armed B-17s into the formation. Our account of the B-17F's wartime role and defensive armament provides wider context for that problem.
The YB-40 was expected to provide a mobile concentration of .50-calibre fire wherever the formation needed it. Calling it a gunship is accurate, although it should not be confused with later side-firing ground-attack gunships: its weapons were intended primarily to defend other bombers against aircraft.
How did the YB-40 differ from a standard B-17F?
The YB-40 traded bombing efficiency and flight performance for substantially heavier defensive armament. A standard B-17F such as the Memphis Belle represented in our FS2004 library is a useful baseline for comparing the two configurations.
| Feature | Standard B-17F | YB-40 escort |
|---|---|---|
| Primary mission | Deliver bombs in daylight formation raids | Protect other B-17s from fighter attack |
| Armament | Defensive gun fit varied by production block and field modification | Extra chin, dorsal and waist firepower, depending on test configuration |
| Ammunition and armour | Balanced against bomb load and range | Far more ammunition plus additional armour and equipment |
| Bomb load | Carried the formation's normal mission load | Normally sacrificed a useful bomb load for the escort fit |
| Performance | Better climb and became lighter after releasing bombs | Heavier, draggier and unable to shed comparable weight over the target |
Typical YB-40 fits are often described as carrying roughly 14 to 16 .50-calibre machine guns, but exact totals differ between sources because the installations changed during testing. Paired waist guns, flexible positions and experimental fits are not always counted consistently.
How can you recognise a YB-40?
The most useful visual clue is the second powered dorsal turret behind the normal upper turret, combined with the chin turret and unusually dense waist armament. A chin turret by itself is not enough: it later became a familiar feature of the B-17G and appeared on some late or modified B-17Fs.
A common identification mistake we see is labelling any chin-turret B-17 as a YB-40. Photographs should be checked for the second dorsal turret and the complete gunship arrangement, not one nose feature.
Did the YB-40 work in combat?
The YB-40 added useful defensive firepower, but its combat trial showed that it could not perform the escort mission effectively enough to justify production. Service-test aircraft flew with the US Eighth Air Force's 92nd Bomb Group from England in 1943 and received combat claims, yet the design had several fundamental disadvantages:
- Weight and drag: extra turrets, armour and ammunition reduced climb and acceleration.
- Formation mismatch: after ordinary B-17s dropped their bombs, they became lighter and faster; the YB-40 remained heavily loaded and struggled to stay with them.
- Limited interception ability: more guns did not give the aircraft the speed or manoeuvrability needed to pursue enemy fighters.
- Added complexity: the extra powered turrets, ammunition feeds and gun positions increased maintenance demands.
- Better alternatives: drop-tank-equipped fighters and longer-ranged types such as the P-38 and P-51 increasingly provided genuine fighter escort.
The decisive problem was therefore not simply unreliable guns or poor tactics. A bomber made heavier to protect other bombers was least able to keep pace at the point when the formation was leaving the target and still vulnerable.
What does the YB-40 designation mean?
The letter Y identified a service-test aircraft, while B placed it in the bomber category. An XB-40 prototype preceded the YB-40 trial aircraft, but the concept never progressed into a standard production B-40 fleet.
Was the YB-40 actually built by Boeing?
Calling it the Boeing YB-40 is conventional because it belonged to the Boeing-designed B-17 family. The original conversion work was undertaken by Vega Aircraft Corporation, then a Lockheed subsidiary, as part of the wider manufacturing system that also produced B-17s through Boeing, Douglas and Vega. Our overview of Boeing's wider aircraft development places the Flying Fortress family in that broader history.
What did the YB-40 contribute to later B-17s?
The escort-gunship concept was abandoned, but parts of the experiment were valuable. Most significantly, the powered chin turret gave much stronger protection against frontal attacks, a weakness Luftwaffe pilots had learned to exploit.
The YB-40 was not simply converted into the B-17G design, but its testing reinforced the value of stronger forward armament. The chin turret subsequently became one of the defining features of the production B-17G, while the YB-40's second dorsal turret and extreme ammunition load were not retained.