The Boeing XB-39

The Boeing XB-39 "Superfortress"

The XB-39 "Spirit of Lincoln" was a modification of the first YB-29 (S/N 41-36954). The XB-39 project was similar to the XB-38 project--the standard air cooled radial engines of the production aircraft were replaced by liquid cooled 'Vee' engines. On the XB-39, the original Wright R-3350-21 Cyclone radials were replaced by Allison V-3420-11 engines. The V-3420 was essentially two V-1710 engine spliced together (one crankcase and two crankshafts geared together).

Click on Picture to enlarge

Pilot

Co-Pilot

Flight Engineer

The XB-39 project was basically a proof-of-concept project to demonstrate performance with liquid cooled 'Vee' engines. It was also insurance against shortages of the production engine. Only XB-39 was built; it was delivered the the US Army Air Force in early 1944 for testing. Most of the problems with the standard B-29 production version were fixed by mid-1944 and no orders for the XB-39 were placed.

TYPE
XB-39

Number Built/Converted
1 (cv)

Remarks
YB-29 with V-3420-17 engines

Notes:

Serial number: 41-36954 (YB-29-BO)

SPECIFICATIONS

Span: 141 ft. 3 in.
Length: 99 ft. 0 in.
Height: 29 ft. 7 in.
Weight: 135,000 lbs. (max. gross weight)
Armament: Ten .50-cal. machine guns plus 20,000 pounds of bombs (max. overload).
Engines: Four Allison V-3420-11 engines of 2,100 hp. each (3,000 hp. ea takeoff power)
Crew: 6 normal; 10 with gunners; 12 with relief crew

PERFORMANCE
Maximum speed: 405 mph. at 25,000 ft.
Cruising speed: 282 mph
Range: 2,850 with 4,000 lbs of bombs; 6,290 miles max. ferry range
Service Ceiling: 35,000 ft.

The first YB-29 (41-36954) was turned over to General Motors for installation of liquid-cooled Allison V-3420 engines and further tests. The converted aircraft was later redesignated XB-39. The V-3420 engine was essentially a pair of Allison V-1710 twelve-cylinder liquid-cooled Vee engines coupled to a single propeller shaft. Normal output was 2100 hp at 25,000 feet. Aircraft speed increased to 405 mph at 35,000 feet, but the improvement in performance was not considered sufficient to justify production.

Joe Baugher

Sources:

Warbird History--B-29 Superfortress, Chester Marshall, Motorbooks International, 1993.

The Boeing B-29 Superfortress, Mich Mayborn, Aircraft in Profile, Doubleday, 1969.

B-29 Superfortress, John Pimlott, Gallery Books, 1980.

Famous Bombers of the Second World War, William Green, Doubleday, 1960.

Boeing Aircraft Since 1916, Peter M. Bowers, Naval Institute Press, 1989.

United States Military Aircraft Since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian Institution Press, 1989.

Jane's Fighting Aircraft of World War II, Bill Gunston, Military Press, 1989.

American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.

The Allison V-3420 Engine

Click on Picture to enlarge

Allison V-3420 Engine

The V-3420 is a 24-cylinder double-V, twin crankshaft, liquid-cooled engine derived from the V-1710, a 12-cylinder engine that powered such WWII aircraft as the Lockheed P-38, Bell P-39, and the Curtiss P-40. Essentially, the V-3420 is made up of two V-1710 engines mounted on a single crankcase with the two crankshafts geared together. It was manufactured from 1941 until 1945 and used in several experimental AAF aircraft including the XB-19, XP-58, XB-39, XP-75, and the P-75A. None of these aircraft were produced in quantity, therefore only about 150 V-3420s were built.

SPECIFICATIONS
Model: V-3420
Type: 24-cylinder, liquid-cooled, double-vee, with gear-driven supercharger and exhaust-driven turbo-supercharger
Displacement: 3,420 cu. in.
Weight: 2,655 lbs.
Maximum RPM: 3,000
Maximum Horsepower: 2,885

Description

a. GENERAL - The XB-39 heavy bombardment airplane is the B-29 airplane with Allison V-3420-17 liquid cooled engines and Curtiss electrically-controlled propellers installed. The primary structure, with the exception of the nacelles, is identical with that of the B-29. The design gross weight is 110,000 pounds.

(1) The XB-39 is a mid-wing monoplane with four 2,100 horsepower engines using four bladed electrically controlled propellers. The wing span is 141' 3”; the length is 99 feet. The overall height is 29' 6.7" (taxiing position). The tricycle landing gear and the tail skid are fully retractable.

(a) FUSELAGE. - The all metal, semi-monocoque type fuselage is divided into main compartments. Three of those compartments can be pressurized so as to provide crew comfort even at extreme altitudes. The forward pressurized compartment and the rear pressurized compartment are connected by a tunnel, through which crew members have access to either compartment during flight. The tail gunners' pressurized compartment is separate.

(b) WING. - The wing consists of a center section, two inboard panels, two outboard panels, and two wing tips. Self-sealing fuel tanks are installed in the inboard wing structure. Both ailerons have trim tabs. Electrically operated wing flaps form the lower surface of the inboard wing trailing edge.

(c) EMPENNAGE. - The empennage is conventional; including a horizontal stabilizer, elevators with trim tabs, a dorsal fin joining the vertical stabilizer, and the rudder with trim tab.

(2) The normal crew of 6 consists of a pilot, copilot, engineer, navigator, radio operator, and bombardier. The alternate crew of 10 adds a top gunner, two side gunners (one may be a radar operator), and a tail gunner. Bunks in the rear pressurized compartment will accommodate two relief crew men. The normal crew members do not have to leave their stations to fly the airplane, but there is ample room for moving around when necessary.

(3) The pilot, copilot, engineer, navigator, radio operator, and bombardier enter through the nose wheel well, and up through the floor hatch near the engineer's station. The gunners board the airplane through the main entrance door which is on the right side of the fuselage near the tail.

(a) Exit is normally made through the doors named in the preceding paragraph.

(4) Shock-mounted, non-magnetic armor plate protects crew members at their stations. In addition, "bullet resistant" windows are provided for the pilot, copilot; and tail gunner.