Boeing XB-39 ...

[WIX_Archive]

The Boeing XB-39 was a single example of the B-29 Superfortress converted to fly with alternative powerplants. It was intended to demonstrate that the B-29 could still be put into service even if the first choice of engine, the air-cooled Wright R-3350 radial engine, ran into development or production difficulties.

Design and development

Starting life as the first YB-29 delivered to the United States Army Air Forces, it was sent in November 1943 to the Fisher Body Aircraft Development Section of General Motors to be converted to use Allison V-3420-17 liquid-cooled W24 (twin-V12, common crankcase) inline engines. Fisher was chosen for the modification as it was familiar with the engine, as it was to power the P-75 Eagle that they were then developing. Testing on it began in early 1944.

Further development of the engine and the aircraft was delayed by a series of changes in the planned turbosuperchargers, as the originally specified GE Type CM-2 two-stage turbosupercharger became unavailable due to demands on GE's production of its other turbosuperchargers. Other turbosuperchargers were considered, but the end result was that the first flights of the B-39 had to be made without any turbosuperchargers at all.

In addition, in early 1944, due to a sudden realization from the U.S. Army Air Forces that it required a long range air superiority fighter, Fisher was directed to focus on its other major project, the P-75 Eagle. In June 1944, Fisher received a contract for 2,500 P-75s. However, in October 1944, for a variety of reasons, the P-75 was canceled.

Operational history

General Motors modified B-29 to use Allison V-3420 engines
Fisher finally focused again on the B-39. The first flight of the B-39 was made on 9 December 1944 at Cleveland, Ohio. The initial flight tests of the B-39, without turbosuperchargers installed, were impressive. However, the B-39 program was by now seriously delayed, and the flawed R-3350 B-29s had already been rushed into combat in June 1944.

Despite continuing problems with the B-29s, the aircraft was functioning well enough in combat that it no longer made any sense to shift resources in the manufacturing base to a new engine for the B-29 and so the B-39 was not ordered into production.

General characteristics

Crew: 10: pilot, co-pilot, flight engineer, bombardier, navigator, radio operator, side gunners (two), top gunner, and tail gunner
Length: 99 ft 0 in (30.18 m)
Wingspan: 141 ft 3 in (43.05 m)
Height: 27 ft 9 in (8.46 m)
Wing area: 1,736 sq ft (161.3 m2)
Empty weight: 74,500 lb (33,800 kg)
Gross weight: 120,000 lb (54,000 kg)
Max takeoff weight: 133,500 lb (60,560 kg)
Powerplant: 4 × Allison V-3420-11 liquid-cooled W24 (double-vee) engines, 2,100 hp (1,600 kW) each

Performance

Maximum speed: 405 mph (648 km/h, 351 kn)
Range: 6,290 mi (10,060 km, 5,460 nmi)
Service ceiling: 35,000 ft (11,000 m)
Wing loading: 69.12 lb/sq ft (337.5 kg/m2)
Power/mass: 0.073 hp/lb (121 W/kg)

Armament

Guns:
8× .50 in (12.7 mm) Browning M2 machine guns in remote controlled turrets
2× .50 in (12.7 mm) machine guns in manual turrets
1× 20 mm (0.787 in) M2 cannon in tail
Bombs: 20,000 lb (9,000 kg)

[Pat Carry]

Interesting Mark, thanks

[Dan Jones]

That kinda seems like it would have been the perfect machine to power with four licence-built RR Griffon engines.

[Noha307]

The Fisher Body Division Aircraft Program Operating Report that our museum digitized a while back (it actually belongs to one of our volunteers) has some information and a few pictures relating the XB-39:

(Source: Fisher Body Division Aircraft Program Operating Report via Ohio Memory)

(Source: Fisher Body Division Aircraft Program Operating Report via Ohio Memory)

XB-39
The XB-39 project consists of designing, building and installing four nacelles for the adaptation to the B-29 (Boeing) Heavy Bomber of the new Allison 3,000 horsepower V-3420 air-cooled engine with 2-stage exhaust-driven turbo supercharger. The work includes building a full-scale mock-up and a ground test nacelle and designing a complete air induction system. This job is 50% accomplished and is scheduled for completion in the fall.

(Source: Fisher Body Division Aircraft Program Operating Report via Ohio Memory)

The XB-39 Project
The XB-39 project adapting four Allison V-3420-11 Engines, with turbo supercharger (2-stage), to the B-29 (Boeing) Heavy Bomber was 57% completed by July 12, 1943, when its progress was interrupted in order to transfer the personnel to the urgent new XP-75 Long Range Fighter program.
A ground-test nacelle had been built, and testing was satisfactorily completed at the Allison plant on June 27, 1943. Four nacelles were under construction, and the forward section of each (back to the monocoque front bulkhead) had been completed and fitted with engine mounts. Parts were being fabricated for the balance of the nacelle structure in anticipation of the delivery to us by the Army Air Forces of a B-29 Bomber.
On November 10, 1943, a YB-29 Bomber, one of a block of 14 semi-production airplanes built at the Boeing plant in Wichita, Kansas, was delivered to our Cleveland No. 2 plant. Work on the XB-39 project was resumed on November 15, 1943, and will be carried to completion as rapidly as possible.

(Source: Fisher Body Division Aircraft Program Operating Report via Ohio Memory)

The XB-39 Heavy Bomber Project
Progress on our XB-39 project --- installing Allison V3420-17 Engines and a Carbon Dioxide Fire-fighting system on a YB-29 (semi-production) Boeing Heavy Bomber --- was severely limited, during the last quarter of 1943 and the early part of 1944, by the enforced concentration of our engineering and technical personnel on the extremely urgent P-75 Program.
We were further hampered by the condition of the wing structure and other sections of the particular Bomber which was delivered to us by the Government. This semi-production airplane, one of the first few assembled at the new Boeing-Wichita plant, was built with a considerable proportion of tool-room parts, and, as a result, we found in the structures affected by our modifications frequent variations from the available drawings. These unanticipated discrepancies cost us time both in reworking parts already in process and in making a thorough engineering survey of large sections of the airplane.
Within the last 90 days, we have been able to press this project forward toward completion at an accelerated rate, using whatever manpower we could spare from the Long Range Fighter activities. More recently, when we were advised by high A.A.F. officials that the XB-39 project had assumed a new and very urgent importance in their planning, we stepped up the work to the utmost practicable limit.
The major design engineering will be completed by the middle of July, 1944; and we are now organizing a priority system to insure the earliest possible completion by processing parts in the exact order of their requirement in assembly.
At the end of June, 1944, the build-up of the center wing had been completed. The structures of two monocoque nacelle assemblies and two engine nacelles were almost finished and their installation was nearing completion. The remaining major assemblies were well along structurally, and installation was under way.
Recently, we have been assigned two new, complicated tasks:
(1) to recover and replace the bomber's armament, which had been removed from our possession by the Government, and
(2) to install an extensive duct system for the cabin superchargers, not in the airplane when it was delivered to us.
Including these, we are now aiming to complete the XB-39 project for flight testing as early as possible in the fall.

(Sources: Fisher Body Division Aircraft Program Operating Report via Ohio Memory (1, 2))


(Source: Fisher Body Division Aircraft Program Operating Report via Ohio Memory)

(Source: Fisher Body Division Aircraft Program Operating Report via Ohio Memory)

(Source: Fisher Body Division Aircraft Program Operating Report via Ohio Memory)

[JohnB]

That kinda seems like it would have been the perfect machine to power with four licence-built RR Griffon engines.

So, an American equivalent of the Shackleton?

[Noha307]

That kinda seems like it would have been the perfect machine to power with four licence-built RR Griffon engines.

So, an American equivalent of the Shackleton?

The relationship between the B-29/B-39 reminded me of the DC-4/Canadair North Star.

[JohnB]

That kinda seems like it would have been the perfect machine to power with four licence-built RR Griffon engines.

So, an American equivalent of the Shackleton?

The relationship between the B-29/B-39 reminded me of the DC-4/Canadair North Star.

It would be interesting to see comparisons both types with, and without, the liquid cooked V-12s.
What the engine swap meant not only to speed, but also range and payload.


For commercial aircraft payload is important, I wonder if that isn't the reason we didn't see more V-12 civil aircraft in the 40s-50s.
Considering the complexity, cost and maintenance requirements that the turbo-compound engines had, the simple expedient of making a Northstar-like DC-7 or Connie must have had serious drawbacks.

[Noha307]

So, an American equivalent of the Shackleton?

The relationship between the B-29/B-39 reminded me of the DC-4/Canadair North Star.

It would be interesting to see comparisons both types with, and without, the liquid cooked V-12s.
What the engine swap meant not only to speed, but also range and payload.

For commercial aircraft payload is important, I wonder if that isn't the reason we didn't see more V-12 civil aircraft in the 40s-50s.
Considering the complexity, cost and maintenance requirements that the turbo-compound engines had, the simple expedient of making a Northstar-like DC-7 or Connie must have had serious drawbacks.

According to Wikipedia, the North Star cruised at 325 mph as compared to 227 mph in the DC-4.

My understanding was that the motivation for the North Star was increased cruising speed, but commercial aircraft designed for high speed have never done well. Whether it's the Convair 880 or the Concorde, it always seems that efficiency wins over reduced travel time in the end. The increased operating costs just end up getting passed onto passengers who would rather have a cheap flight than a fast one. (cf. the rise of low-cost carriers) It's apparently just not worth it. Now that there's supersonic business jets in the offing maybe we'll finally break that trend, but I have my doubts.

The other thing is that - whether it's sonic booms or engine exhaust - fast aircraft tend to be LOUD and the North Star was no exception. I imagine any other inline engine transports might have had the same problem. One thing I don't know, and maybe you have the answer to, is how did power-recovery turbines affect the volume of the exhaust on the R-3350?

I have to agree though, a Connie with inlines would be wicked!

Last edited by Noha307 on Tue Jun 29, 2021 11:47 am, edited 1 time in total.

[JohnB]

While it has nothing to do with the relative benefits and efficiencies of the types, in his book Douglas Aircraft, Francillion mentions another consideration in the sale of Northstars, at least to some operators was buying a Canadian built aircraft with UK-built engines, BOAC could avoid import duties.

[Dan Jones]

My suggestion of using a licence-built version of the Griffon engine was just based on simplicity and reliability. None of the X type engines (the -3420, DB606, RR Buzzard, etc) ever seemed to have really amounted to anything apart from being ticking bombs. The Griffon, though of slightly lesser power, would have been much more reliable and probably lighter (and the powerplant probably more Lincoln like than Shackleton).