Today
we’re taking a look at the Boeing XB-15, which was considered to be the first of the so-called
“super bombers” that were developed in the United States in the lead up to,
and during the second world war. It can be considered the cousin to the Douglas XB-19,
which we covered recently, but although these two aircraft were both incredibly large, setting
records in their own right, the Boeing has often been overshadowed or outright ignored – despite
its own contributions to the field of designing large aircraft. Today’s video attempts to correct
this, though sadly there’s nowhere near as much information available on the XB-15 as there was on
the Douglas XB-19, so it’s a little bit shorter. Its also a reminder of the powerful role media
plays in shaping public opinion and why I’m happy to talk about
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sponsoring today’s video, and now lets get back to the Boeing XB-15.
Like the recently featured Douglas XB-19, the Boeing XB-15 can trace its origins back to the
early 1930s. Aircraft technology was advancing at a rapid pace, and with the emergence of the
all-metal cantilever monoplane, the US Army Air Corps wanted to explore the development of very
large, very long-range bombers. Considering the geopolitical and geographical situation,
this made a lot of sense. Any nation that could realistically be expected as a future adversary,
in some future war, was incredibly far away. With their current technology, any future war would
require the United States to base their air units on the territory of a future ally -say,
for example, the United Kingdom, if war were to erupt with the Soviet Union.
But if the US were denied the luxury of basing their bomber force on friendly soil,
or, perhaps if they were to fight a war in the Pacific, with thousands of miles separating
their scattered territories, they would struggle to mount any kind of effective aerial campaign,
as nothing in their inventory had the range. This resulted in the issuing of Projects A and D
of the Experimental Long Range Bomber program. Project A initially called for a design that
could carry 2,000 lbs (910 kg) of bombs over a 5,000 miles (8,046 km) range at a speed of
200 mph (320 km/h). The bombload was eventually deemed totally inadequate, and the requirement
was at some point scaled up, though there seems to be little source material on what the new,
larger bombload requirements actually were. Eventually, four companies responded to the Army’s
requirements: Boeing and Martin submitted designs for project A, while Douglas and Sikorsky drew up
designs for the more ambitious project D. Known internally as the model 294,
and initially labelled by the Army as the XB-15, the Boeing design was drawn up by a team led by
John “Jack” Kylstra. It was a considerable step up from any aircraft previously built by the company;
its two most ambitious designs up until that point had been the twin-engined 247 airliner
and the Y1B-9 experimental bomber. The model 294 on the other hand was to be a four engine
aircraft whose wingspan alone was twice that of either of those designs, and it was to feature a
large suite of cutting edge technology. In the subsequent Air Corps review,
which compared the proposals from Martin and Boeing, the design submitted by Boeing
was viewed to be superior and it was ordered to be built as the XB-15 prototype. Not long after this,
Projects A & D were amalgamated into the new Experimental Bomber Long-Range program,
and the XB-15 became known as the XBLR-1. Much like the XBLR-2, which became the Douglas
XB-19, completion of the Boeing design faced significant delays. In fact, it was some of
these delays – as the design preceded the Douglas model by some 18 months- that almost led to the
cancellation of the XB-19 by Douglas, had the Air Corp not stubbornly dug their heels in.
Not only was producing an aircraft on this scale a significant challenge for Boeing,
as nothing before it came close in scale, but finding engines powerful enough to
get it airborne was even more so. Due to its projected size and weight, Boeing had planned
to use the experimental V-3420 being designed by Allison. Comprising of two V-1710 engines,
which were stuck together on a common crank case, this 56 litre, W-24 was expected to put out over
2,000 hp at take-off – indeed, it would eventually manage 2,600. But this engine was not ready when
Boeing needed it, and they were forced to fall back on much less powerful alternatives.
Eventually, the Pratt & Whitney R-1830 Twin wasp was selected instead. And while on its own
merits it was a very good engine, the model being installed on the Boeing prototype only produced
1,000 hp at take-off, which was a significant downgrade.
On its own this was trouble enough, as having your shiny new bomber being reduced to less
than 40% of its intended power was less than ideal, but then the Boeing design
team went through the painful process of actually building the aircraft as well.
Construction of the prototype took over two years to complete, with it not being ready until the
Autumn of 1937. Though the powerplant issue had caused some of this delay, most of it was a result
of the aircraft’s size. Entirely new construction practices, tools, jigs, and installation
procedures were pioneered, and pioneering this stuff -with its usual routines of trial and
error- took time. In fact it took so long that by the time the prototype was complete, the XBLR
program had been dropped, and the aircraft would make its first flight as the XB-15 once again.
A good example of the challenges that were faced, and overcome, with the XB-15 is to compare it to
the Model 299. Design work on the 299 had began at almost the same time as that for the XB-15,
with the Air Corps requesting a replacement for the Martin B-10, but the XB-15 had enough of a
head-start that the lessons learned from its own development could be applied in real-time
as construction of the 299 got underway. Because of this, the 299, the prototype
for the B-17 flying fortress, made its maiden flight two years before the XB-15. Its smaller
size certainly helped to speed up the process, but many aspects of its construction had been
directly improved by the growing mountain of challenges that were dogging that of the XB-15.
Eventually, said challenges were overcome, and the Boeing team breathed a sign of relief as the XB-15
rolled out for its maiden flight on the 15th of October 1937. At the time it was the largest and
most advanced aircraft built in the United States. It had a wingspan of 149 feet (45.4 meters),
a length of 87 feet 7 inches (26.7 meters), and a height of 18 feet 1 inch ( 5.5 meters). Much
like the XB-19, The wings were so large that they allowed for the provision of a
service tunnel, which allowed engineers access to the engines during flight to
perform maintenance and emergency repairs. Empty it weighed approximately 37,700 lbs (or
17,100 kilos) and fully loaded it could take off at 70,000 (or 32,000 kilos). As a result of
the heavy weight, it had been decided to use twin wheels for each of the main undercarriage units,
something that might have saved the designers of the literally ground-breaking Douglas XB-19 some
considerable grief, had they done the same. Due to the long range it was intended to fly,
the aircraft featured a cutting-edge autopilot system, along with spacious accommodation for
the crew. In fact it was remarked that the XB-15, with its upholstered crew resting area,
complete with bunks, lavatory facility, and a galley, was more luxurious than
many transatlantic airliners at the time. Technical innovations included -but were not
limited to- a new fuel system, de-icing systems for the wings, electrical power was supplied by
two 100-volt AC generators, which themselves were driving by two auxiliary petrol engines,
and the aircraft featured a station for a flight engineer – who took over many of the
mechanical duties that would have gone to the pilot and co-pilot, which for an aircraft of
the XB-15’s size and complexity would have been a considerable task ontop of their other duties.
Though all of this was deeply impressive, and it was the most advanced aircraft Boeing had yet
produced, the XB-15 was already outclassed before it had even left the ground. This was the price
of progress. Firstly, the numerous delays caused by pioneering the XB-15s construction meant that
it was completed months, or even years after other aircraft were already benefiting from the lessons
it had produced. Secondly, work was progressing, albeit at a glacial pace, on the Douglas XB-19,
which dwarfed the Boeing design in physical size, range, payload capacity, and of course
weight. And thirdly, the projected requirements for a heavy bomber had changed so much in just
a few short years, that bringing the XB-15 up to scratch would require a major re-design.
Though sometimes referred to as the “Super Flying Fortress” the XB-15 had considerably
less defensive guns than the B-17. It was equipped with just six: three .30 calibre machine guns,
and three .50 calibre. Two guns were mounted in ventral turrets, one in a dorsal turret,
another was in the nose, and there were two waist positions – a tail gun was not provided at all.
When first designed, this was the most powerful defensive arrangement yet seen on a bomber,
but again by the time the XB-15 was complete it could only be deemed as “adequate”. Just.
The question of obsolescence was further confirmed during the aircraft’s maiden flight,
and subsequent evaluations. It was flown by test pilots Major John Korkille from the Air Corps,
and Edmund Allen (an engineer who often worked with Boeing). Though they reported
that the XB-15 handled well for its size, and was considered easy to manoeuvre, its power
deficit resulted in some very disappointing performance figures. Even when empty,
it could not attain the required speed of 200 mph, topping out 197,
and when loaded with a simulated bombload of 2,000 lbs this dropped down to just 145 mph.
To say this ruled out any possibility of the XB-15 being developed into an actual frontline
bomber would be an understatement, however all was not lost. Though not particularly fast,
it could still carry large payloads, far more than the 2,000lbs initial tested, and there were hopes
of a change in powerplant, and Boeing were looking at follow-up designs as well. Because of this,
and not wanting to waste valuable resources, time, and money, the XB-15 was officially entered
into Amy Service in 1939, being assigned to the 2nd Bombardment Group based at Langley Field.
The planned successor for the XB-15 design was known as the Y1B-20, or the Boeing model 316.
It was meant to be slightly larger than the XB-15, and it was to have much more powerful engines. Two
orders were placed at some point in late 1938, but these were eventually cancelled in favour of other
projects: specifically the one that would lead to the B-29. Part of the XB-15 design would live on
in the Boeing 314 Clipper, which literally took the design of the XB-15’s wing and essentially
slapped it onto the hull of a giant flying boat. As for the XB-15. It’s service life was relatively
brief, but it was quite eventful. It earned a moderate claim to fame during the events
following the 1939 Chilean earthquake. Flown by a crew led by Major Caleb Haynes,
the XB-15 was flown down to Santiago for an urgent relief mission carry supplies for the Red Cross.
Thanks to its huge range, it only had to make two stops along the way -at Panama, and Lima-;
after completing this vital aid mission, Haynes was awarded the Order of the Merit of Chile,
and would also receive the distinguished flying cross. The flight, carried out in record time,
across remote regions, at near record ranges, also earned the entire crew the MacKay Trophy
– an annual award given in the United States for the most meritorious flight of the year.
Though the XB-15 was inadequate as a bomber, this flight, and subsequent flight tests throughout
1939, proved that it could operate as an excellent transport. Throughout the year,
several payload and altitude records were set by the XB-15, along with some for endurance as well.
On August the 2nd, the XB-15 set a world record for flying 3,109 miles, with a 4,400 lb payload,
at an average speed of 166.3 mph. Because of this, the combat load for the
XB-15 increased to approximately 8,000 lbs. And although its usefulness as a bomber was limited
due to its speed, it did still drop the occasional bomb, albeit in training exercises and testing,
rather than as a weapon of war. By the time of Pearl Harbor it had actually been
stripped of all defensive weaponry and was used exclusively for bomb
accuracy tests and various transport duties. With the United States now at war with Japan, its
use as an experimental bomber came to an abrupt end, and it was decided to convert the XB-15
into a dedicated transport. A set of cargo doors were installed on the fuselage, as was an internal
cargo hoist, and the aircraft was redesignated as the XC-105 on the 6th of May 1943.
Operated by the 20th Troop Carrier Squadron, the XC-105 was often nicknamed “Grandpappy”.
This went against the trend of often giving aircraft and naval vessels feminine names
and titles, and this may have been a reflection on the somewhat recalcitrant
nature that had taken hold of the aircraft since its conversion to transport duties.
It’s gross weight had gone up to 92,000 lbs, and along with the addition of extra cargo equipment
and hoists, this had a horrible effect on its in-flight handling. While the XB-15 was found to
be relatively easy to fly for an aircraft of its size, the XC-105 flew with the grace of a startled
elephant: heavy, and prone to erratic movements. This may have had a part in the aircraft’s
retirement after just 18 months of use in this role, but there was also the issue of getting
spare parts for a one-off airframe, and the fact that the US was building huge numbers of dedicated
transport aircraft as it was. The issue of spare parts was further exacerbated by a complete
failure of the electrical system, which occurred mid-flight, and at least two separate incidents
where the aircraft caught fire – possibly as a result of said electrical problems,
but the details on this have been hard to find. Still, despite trying to electrocute and
immolate its six-man crew, the XC-105 completed at least 70 transport missions,
and an almost equal number of other missions such as anti-submarine reconnaissance. It
transported over 5,200 passengers, 440,000 lbs of cargo, and almost 100,000 lbs of mail.
After being retired from use in December 1944, the XC-105 was deemed unsafe for further flight and
it was assigned to the Panama Air Depot. There it lay for approximately six months before scrapping
began, with the engines, ruder and vertical stabiliser being remove, as well as various
internal parts and equipment. The empty shell that remained was then unceremoniously dumped
in a swamp southwest of the airfield…where it still lies today, covered by an industrial area.
The XB-15 got a pretty rough treatment, both at the end of its life, and in the eyes of the US
Army Air corps, and those who study aviation history. A large part of this was because
it was literally overshadowed by the Douglas XB-19, which was developed slightly after it,
but was already benefiting from improvements in technology – specifically in the design of
powerplants. But the XB-15 was still vitally important to Boeing, and its development aided
in the establishment of practices that were used to produce the B-29 Superfortress..
As always thank you all so much for watching, and a big thank you of course to the Patreon
supporters who’s names you’ll see here. This video should have been up a week ago but as
usual about a dozen things got in the way, seems to be a running theme at the moment,
but hey, here we are. I’ve also had issues with my voice which hasn’t helped.
A big thank you of course to our wing commander tier patrons, out highest tier supporters,
and a warm welcome to Rhys Revs who is the newest member of this special group.
I’ll be putting an update on Patreon soon about the planned trip to the UK,
and a couple of things I’m hoping to do whilst I’m over there, so keep an eye out for that.
But thank you all for your continued support, and I’ll catch you all next time. Goodbye.