From the Planes (and Perhaps Engines) You’ve (Probably) Never Heard Of Department of Wingspan, we bring you the Boeing XB-39 Superfortress.
When the Boeing B-29 Superfortress took its maiden flight on September 21, 1942, it was one of the largest and most technologically advanced heavy bombers of its day. It would go on to be one of the most effective bombers of the war, and enjoy a successful postwar career where it served as the basis for numerous other aircraft, both military and civilian. But early problems with the reliability of its radial engines caused major headaches for the Army Air Forces, and they began looking at other possible engine solutions. They also wanted a fallback in case the supply of radial engines was interrupted. So, like the XB-38 Flying Fortress before it that experimented with inline rather than radial engines, a radical new inline engine was also tested for the B-29.
Liquid-cooled V-12 engines were all the rage in Europe, but the United States fielded only a single V-12 aero engine during WWII, the Allison V-1710 (a number which reflects the engine’s displacement). This engine appeared in a number of iconic warbirds, including the Lockheed P-38 Lightning, Curtiss P-40 Warhawk, and North American F-82 Twin Mustang. But if 12 cylinders are good, wouldn’t 24 cylinders be even better? The result of that thinking was the Allison V-3420 24-cylinder engine.
At its most basic level, the V-3420 is two V-1710s placed side-by-side (because 1710 x 2 = 3420), with the two crankshafts joined through a common crankcase. The “double-v” configuration resulted in what was known as a W engine (because, after all, a W is two V’s, not two U’s). Fitted with both a gear-driven single-stage supercharger and a turbocharger, the V-3420 produced 2,600hp at full military power while turning at 3,000 rpm, and 1,575hp while cruising at 25,000 feet. Not only was the experimental engine fitted to the XB-39, it was also used in other experimental aircraft such as the Douglas XB-19, Fisher P-75 Eagle and Lockheed XP-58 Chain Lightning.
Boeing loaned the first pre-production YB-29 (41-36954) to General Motors, who undertook the task of mating the massive V-3420s to the bomber. With the increased power, the XB-39, named Spirit of Lincoln, reached a top speed of 405 mph at 25,000 feet—about 50 mph faster than the radial engined B-29 and just 35 mph slower than the top speed of the Mustang. It could top out at 35,000 feet, about 3,000 feet higher than the B-29. Cruising speed was slightly slower than the B-29, but the maximum range was more than 6,000 miles.
The XB-39 took its maiden flight on December 9, 1944 and, by all accounts, the Allison engines performed well and the overall performance was impressive. However, despite continuing problems with the radial engines of the production B-29s that were already in service, the Army Air Forces decided that the bombers fighting in the Pacific were doing well enough that it made little sense to completely overhaul the production line to introduce the new engine. And with so much attention being paid to the development of the XP-75 Eagle and its V-3240 engine, there was little time to spend perfecting the engines for use on the XB-39. Only a single XB-39 was built, and its final disposition is unknown.
Much more information about the Allison V-3420 can be found here.