Boeing XB-15

https://en.wikipedia.org/wiki/Boeing_XB-15

The Boeing XB-15 (Boeing 294) was a United States bomber aircraft designed in
1934 as a test for the United States Army Air Corps (USAAC) to see if it would
be possible to build a heavy bomber with a 5,000 mi (8,000 km) range. For a year
beginning in mid-1935 it was designated the XBLR-1. When it first flew in 1937,
it was the most massive and voluminous aircraft ever built in the US. It set a
number of load-to-altitude records for land-based aircraft, including carrying a
31,205 lb (14,154 kg) payload to 8,200 ft (2,500 m) on 30 July 1939.

The aircraft's immense size allowed flight engineers to enter the wing through a
crawlway and make minor repairs in flight. A 5,000 mi (8,000 km) flight took 33
hours at its 152 mph (245 km/h) cruising speed; the crew was made up of several
shifts, and bunks allowed them to sleep when off duty.

Design and development

The specification that produced the XB-15 began in mid-1933 as "Project A",
USAAC discussions regarding the possibility of flying a very large bomber with a
range of 5,000 mi (8,000 km). In April 1934 the USAAC contracted with Boeing and
Martin to design a bomber capable of carrying 2,000 lb (910 kg) at 200 mph (320
km/h) over a distance of 5,000 miles. Boeing gave the project the internal name
of Model 294, while the USAAC called it the XB-15. Martin's design, the XB-16,
was judged inferior by the USAAC before a prototype was built, and was canceled.

The Boeing design team, headed by Jack Kylstra, initially intended the aircraft
to use 2,600 hp (1,900 kW) Allison V-3420 liquid-cooled W engines; since these
were not ready, 850 hp (630 kW) Pratt & Whitney R-1830 air-cooled radial engines
were used instead.

In mid-1935, the USAAC combined Project A with Project D; a proposal asking for
"the maximum feasible range into the future." The combined program was
designated BLR for "Bomber, Long Range". The XB-15 was renamed the XBLR-1; it
was joined under the BLR program by two other projects: one from Douglas
Aircraft, the XBLR-2 which later became the XB-19; and one from Sikorsky
Aircraft called the XBLR-3, later canceled. The next year, the XBLR was dropped
and the Boeing prototype was once again the XB-15.

Unusual features that the XB-15 pioneered included an autopilot, deicing
equipment, and two gasoline generators used as auxiliary power units
(independent of the main engines) to power the 110-volt electrical system. The
main engines were serviceable in flight using an access tunnel inside the wing.
The aircraft contained a sizable crew compartment with bunkbeds, a galley and a
lavatory. Finally, in September 1937 construction was finished, and on 15
October it first flew. Its double-wheel main landing gear remained down from
takeoff to landing. On 2 December 1937, the XB-15 flew from Seattle to Wright
Field in Ohio to be accepted by the USAAC for testing.

With the Twin Wasp radial engines installed — the same number and type of
engines fitted to the later Consolidated B-24 Liberator, with individual
turbochargers added on the Liberators' Twin Wasp powerplants — the specified
speed of 200 mph for the Twin Wasp-powered XB-15 was not quite reached even when
the aircraft was empty; the best speed attained in level flight was 197 mph (317
km/h). Loaded with the specified 2,000 pounds, the maximum speed was a
disappointing 145 mph (233 km/h). This was considered too slow for a combat
aircraft, and the project was abandoned. However, Boeing engineers projected
that the prototype would be capable of carrying the heaviest air cargo to date:
a load of 8,000 lb (3,600 kg).

The design challenges stemming from the great size of the XB-15 were difficult
to master, but the lessons learned by Boeing were later applied to the Model 314
flying boat, which essentially used the XB-15's wing design with four of the
more powerful Wright Twin Cyclone fourteen-cylinder radials for power. In 1938,
the USAAC proposed to update the XB-15 to make the slightly larger Y1B-20, again
using four Wright Twin Cyclones as with the Boeing 314, but the Secretary of
War, Harry Hines Woodring, canceled the project before construction began, in
favor of the expensive Douglas XB-19. Boeing went ahead with an internal
redesign of the XB-15 called Model 316, a very heavy bomber with a high wing, a
pressurized cabin and tricycle gear. The Model 316 was not built. The
progression of design work starting with the XB-15 finally bore fruit with the
Model 345 presented to the USAAC in May 1940, the very heavy bomber which
resulted in the USAAF's Boeing B-29 Superfortress.

Role
Heavy bomber

Manufacturer
Boeing

First flight
15 October 1937

Status
Canceled

Primary user
United States Army Air Corps

Number built
1 prototype

Developed into
Boeing Y1B-20

Operational history

The single prototype was assigned to the 2nd Bombardment Group at Langley Field,
Virginia. Following the 24 January 1939 Chillán earthquake in Chile, the
prototype flew a relief mission, carrying medical supplies. Commanded by Major
Caleb V. Haynes, the aircraft carried 3,250 lb (1,470 kg) of American Red Cross
emergency supplies to Santiago, making only two stops along the way, at France
Field in the Panama Canal Zone, and at Lima, Peru. Haynes was awarded the
Distinguished Flying Cross and the Order of the Merit of Chile, and the whole
crew earned the MacKay Trophy.

Flying from Langley, the XB-15 arrived at Albrook Field in Panama on 10 April
1940 and immediately began classified bombing tests of canal lock protections,
commanded by Haynes and including Captain Curtis LeMay as navigator and
Lieutenant John B. Montgomery as bombardier. Of 150 bombs dropped, only three
hit the target: a specially made bunker simulating a reinforced machine room.
The few hits nevertheless led to improvements in bunker design. In early May,
Haynes and LeMay made a survey flight from Panama over the Galapagos islands,
the inspection including Baltra Island. Haynes piloted the XB-15 back to the
United States, leaving Panama on 11 May 1940.

In late 1940 the XB-15's defensive guns were removed at Duncan Field in Texas.
Seats were attached so that Lend Lease aircraft ferry crews could be returned
after delivery.

Cargo aircraft

On 6 May 1943 the Army Air Forces converted the only prototype into a transport,
the aircraft being redesignated XC-105. A cargo hoist was mounted, and cargo
doors fitted. Its maximum gross weight was increased to 92,000 lb (42,000 kg).
By this time, the aircraft was nicknamed "Grandpappy" by 20th Troop Carrier
Squadron airmen. It displayed nose art depicting an elephant carrying a large
crate on its back labeled "supplies". During World War II, the XC-105 carried
freight and personnel to and from Florida, and throughout the Caribbean, based
out of Albrook Field beginning in June 1943. Hundreds of young women were flown
in "Grandpappy" from Miami to the Canal Zone to engage in US government work;
these trips were dubbed the "Georgia Peach Run". "Grandpappy" traveled to the
Galapagos, landing on Baltra Island at the same airfield built following the
XB-15 aerial survey of May 1940.

"Grandpappy"'s flight crew, reduced to six men, described the aircraft as
difficult to fly and service. Two fires and a complete failure of the electrical
system occurred in the air. The aircraft was retired on 18 December 1944,
assigned to Panama Air Depot. In June 1945, it was ordered to be scrapped at
Albrook Field in Panama, its engines and internal parts removed along with its
vertical stabilizer and rudder. The remaining airframe was deposited at Diablo
dump, a swampy landfill southwest of the runway, where it slowly sank from
sight. Squatters built shacks on stilts in the swamp, covering the remains. The
former dump is now an industrial area, with "Grandpappy" underneath.

During its 18 months of transport service, the XC-105 carried more than 5,200
passengers, 440,000 lb (200,000 kg) of cargo and 94,000 lb (43,000 kg) of mail.
It flew 70 cargo trips and 60 missions including anti-submarine patrol.
Unusually, the aircraft was consistently referred to as "he" by its crew.

Specifications (XB-15)

General characteristics
Crew: ten
Length: 87 ft 7 in (26.70 m)
Wingspan: 149 ft 0 in (45.43 m)
Height: 25 ft 10 in (7.87 m)
Wing area: 2,780 sq ft (258.4 m2)
Empty weight: 37,709 lb (17,141 kg)
Max takeoff weight: 70,706 lb (32,139 kg)
Powerplant: 4 × 14-cylinder Pratt & Whitney R-1830-11 radial engines, 1000
(take-off) 850 hp @ 6,000ft hp (634 kW) each

Performance
Maximum speed: 197 mph (317 km/h, 171 kn) at 5,000 ft (1,500 m)
Cruise speed: 152 mph (245 km/h, 132 kn) at 6,000 ft (1,800 m)
Range: 5,130 mi (8,260 km, 4,460 nmi)
Combat range: 3,400 mi (5,474 km, 2,957 nmi)
Service ceiling: 18,900 ft (5,760 m)

Armament

Guns:
3 × .30 in (7.62 mm) M1919 Browning machine guns
3 × .50 in (12.7 mm) M2 Browning machine guns

Bombs: 12,000 lb (5,400 kg)


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