Military Aviation

The Boeing YB-40: how the B-17F strategic bomber was turned into a four-engined escort fighter

The YB-40 could carry nearly triple the amount of ammunition – 11,200 rounds as compared to 3,900 rounds carried on a B-17F.

In the early years of the Allied Combined Bomber offensive in Europe, before long-range fighters like the P-51D became available, the bombers of the 8th Air Force suffered mightily at the hands of the German fighters. Because of those losses, the US Army Air Forces (AAF) devised plans to modify bombers with additional defensive armament. As told by Jim O’Connell, 92nd Air Refueling Wing Historian, in the article Boeing YB-40 Flying Fortress, the modified bombers would accompany regular bomber formations and provide protection from the attacking German fighters all the way to the target and back. The 92nd Bombardment Group was the only unit to fly the Boeing YB-40 “gun-ships” in combat.


In August 1942, the AAF contracted with Vega Aircraft Corporation to modify the second production B-17F to become the first XB-40 (‘X’ for experimental) and later renamed the YB-40 (‘Y’ for service test) gunship. Initially, the AAF ordered thirteen 13 YB-40s. Modifications included converting the bomb bay into an ammunition magazine and installing additional armor plating to protect the various crew positions.

The YB-40 could carry nearly triple the amount of ammunition – 11,200 rounds as compared to 3,900 rounds carried on a B-17F. Additional modifications included adding a chin turret with two .50 caliber light-barrel machine guns, replacing the single .50 caliber waist guns with staggered, twin-mounted emplacements and most notably adding a second manned dorsal turret installed in the former radio compartment. The YB-40 had 18 or more guns as compared to only 13 guns on the standard B-17F.

YB-40 cutaway

In May 1943, 12 YB-40s joined the 327th Bombardment Squadron, 92nd Bombardment Group at Royal Air Force Base Alconbury for operational tests. The thirteenth aircraft ran out of fuel and crash landed on an island near Scotland. The YB-40s flew their first operational mission on May 29, 1943 in an attack against submarine pens and locks at Saint-Nazaire. The leadership quickly discovered that the YB-40s were unable to keep up with the B-17s, especially on the return from the target after the formation dropped their bombs. The YB-40s became more of a burden than a help to the formation as they fell behind. The YB-40s were nearly 4,000 pounds heavier than the standard, fully armed B-17.

Also, the YB-40 had significantly greater aerodynamic drag due to the additional gun stations. As a comparison, it took the B-17F only 25 minutes to climb to an altitude of 20,000 feet while it took the YB-40 48 minutes to climb to the same altitude. After participating in only ten missions, in August 1943, the YB-40s were withdrawn from service. A few months later, the first long-range fighter escorts with the P-51B and later P-51D Mustang began operations in Europe and eventually neutralized German fighters.

This print is available in multiple sizes from – CLICK HERE TO GET YOURS. B-17G Flying Fortress – 42-31076, LG-V “Chief Sly’s Son” 91st BG, 322nd BS – 1944

While the YB-40 concept proved a failure with the aircraft being too heavy and slow, the crews from the 92nd Bombardment Group provided some valuable lessons from the operational testing of the YB-40s. These lessons led to modifications conspicuous to the final production variant of the B-17 bomber, the B-17G as well as to the late production B-17Fs. These B-17s incorporated the chin turret, the offset waist gun positions which allowed for greater freedom of movement, and an improved tail gunner station with much larger windows. As a result, the later production B-17Fs and B-17Gs were more effective in defending themselves.

Photo credit: U.S. Air Force

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

View Comments

  • Question for Dario Leone about the YB-40: Can you get me e-copies of any period documentation describing the issue the YB-40 had keeping in formation with B-17Fs after bomb drop? Looking for an memos written by COL Reid (92BG CSR) that explain this issue, and also maybe a copy of a memo I’m told he wrote stating he did not want any more YB-40s. Maybe even copies of mission logs? I think I understand the drag/throttle/fuel consumption but want period documentation to make the point for Simone else in a B-17 fan group.



Recent Posts

B-52 bomber Makes Emergency Landing due to Engine Fire, No Crew Injured

B-52 Emergency Landing Early on Feb. 23, 2024 a B-52H bomber at Minot Air Force… Read More

17 hours ago

Here’s why Four Weeks were required to Scuttle USS America, the Only Supercarrier Ever Sunk

USS America USS America (CVA/CV-66) was one of three Kitty Hawk-class supercarriers built for the US Navy in… Read More

2 days ago

Naval Aviator explains why the F-35 and its incredible capabilities would have made Top Gun: Maverick boring

The F-35 The 5th generation F-35 Lightning II integrates advanced stealth technology into a highly… Read More

3 days ago

The “Seahorse:” the P-51D Carrier Testing and why the Mustang was found unsuited for service aboard US Navy flat tops

A modern aircraft carrier By the 1920s, the quest for a modern aircraft carrier was… Read More

3 days ago