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B-17 Modification Overview


The B-17 is one of the iconic allied bombers of WWII. First flown in 1935 and in service up until 1968 in the Brazilian Air Force, the B-17 was a sturdy, heavy lifting power house. Practically anyone familiar with military aviation will be familiar with the B-17, but what many are not aware of are the interesting and unique B-17 modifications that were constructed during WWII. The most notable B-17 modifications were the XB-38, The YB-40 and the C-108.



The XB-38


The XB-38 was a converted B-17 modified to mount 4 Allison V-1710 engines. This replaced the usual complement of 4 Wright R-1820 radial engines.

The XB-38 was built in 1943 and first flew in May of that year. It was capable of a higher top speed (287 mph increased to 327 mph) but saw a decrease in it’s service ceiling(35,600 feet to 29,600 feet.) The project was canceled due to a number of accidents, including a serious engine fire that resulted in the destruction of the only prototype. Additionally, the Allison V-1710 engine was needed for production of the P-38 Lightening and the P-51 Mustang, among others. In 1944 a similar project was started to install inline engines on a B-29 Superfortress. Called the XB-39, the modified aircraft performed well enough, but cost concerns prevented the new design from entering production.


YB-40

Introduced in 1943, before the P-51 arrived in Europe for high altitude escort missions, the YB-40 was B-17 modified to act as an escort gunship for bomber formations penetrating into the European continent.

While the normal B-17 carried 13 Browning 50 caliber Machine guns, the YB-40 carried, on average, 18 and had room for up to 30 machine guns of various calibers. Some YB-40s even carried guns up to 40mm. Additionally, the YB-40 was full to the brim with ammunition, allowing it to sustain fire for far longer than the normal B-17. The YB-40 carried approximately 10,700 rounds of 50 caliber ammunition with 4,000 rounds stored in the bomb bay.

Compare this with the normal B-17 which carried approximately 2,000 rounds total. The trade off was, of course, speed, and climb rate. The YB-40 was reported to take 48 minutes to climb 20,000 feet while the bombers it was escorting only took 25 minutes to climb to the same altitude. 

This drawback, along with the fact that the YB-40 could not keep up with the B-17 formations, especially after they had dropped their bombs, meant that the YB-40 did not see use after the introduction of the P-51 Mustang. Despite its drawbacks, the 25 completed YB-40s flew 48 sorties and shot down 5 German aircraft plus 2 probables. The last recorded YB-40 combat mission was in July of 1943, however, the new turrets developed for the YB-40 such as the Bendix chin turret (pictured below) and the improved “Cheyenne” tail turret proved effective enough to be mounted on late war B-17 bombers.

A single Consolidated B-24 Liberator, called the XB-41, was modified in a similar fashion to act as a long range bomber escort, but it never saw combat.


XC-108

The XC-108 was a modified B-17 bomber converted to act as a V.I.P. transport for General Douglas MacArthur.

The plane was stripped of arms and armour except for the nose and tail turrets. The interior was converted into a private office for the General. The plane was fitted with a kitchen and living space. After the success of this model, the USAAF, with plenty of obsolete bombers laying around, began looking into ways to convert bombers into transport craft. A number of models were produced including the XC-108A, cargo and troop transport that could carry up to 64 fighting men, and the XC-108B which was converted into a fuel tanker. The B-24 Liberator was similarly modified into a transport known as the Liberator express. This model saw widespread use with 287 aircraft built.


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