[image from the San Diego Air and Space museum]
With the production lines having problems with the B-29 itself as well as its complex R-3350 engines it should be no surprise that alternates to each were looked for. We have already looked at replacement bombers so now lets take a look at a replacement engine option for the Superfortress.
Unlike replacing the engines on the Flying Fortress with V-1710’s this had a reason for being. The Wright R-3350 was having its own teething problems. Plus there was a real question as to whether enough of them could actually be produced to power all the B-29’s needed to fight the war and inevitable invasion of Japan.
Into this void steps our friend the Allison V-3420 coupled engine. This engine is at the heart of a lot of failed prototypes. The Allison inline watercooled twenty four cylinder engine is actually two of the standard V-1720’s coupled together. This makes this XB-39 effectively an eight engine bomber. Remember this was meant as a way to lower the complexity of the air cooled R-3350 engines.
[image National Air Force Museum]
- Maximum speed: 405 mph (351 knots, 648 km/h)
- Range: 6,290 mi (5,460 nm, 10,060 km)
- Service ceiling: 35,000 ft (11,000 m)
- Wing loading: 69.12 lb/ft² (337.5 kg/m²)
- Power/mass: 0.073 hp/lb (121 W/kg)
During war time every day matters in aircraft development. Although they were able to get the B-29 working with the coupled engines it was already past the time that Wright needed to iron out the R-3350’s issues. So the project never moved forward past the prototype.