It was an initial bad design by engineering. Killed quite a few people including one of Boeings most revered test pilots to this day, Eddie Allen. May he, his crew, and those on the ground rest in peace.
"The most relentless problem was the 2,200 hp Wright Cyclone R-3350 twin row radial engine. It had a persistent inclination to overheat, swallow valves and even catch fire in flight. In an effort to produce more horsepower from a lighter engine, the crankcase was made of magnesium, a very light, very strong metal. The problem was magnesium is also a flammable metal. When that was combined with the additional problem of a fuel induction system, which tended to catch fire and burn long enough to catch the magnesium afire, it became a very serious situation. "Band-Aid" treatments such as air baffles to direct more air to the rear row of cylinders and propeller cuffs to force more air through the engine helped, but it would be many moons before the problem was solved. Boeing lost its' Chief Test Pilot along with the cream of the B-29 flight test crew because of a fire which destroyed a wing spar. Shortly after noon on Thursday, February 18, 1943, Eddie Allen was flight testing the number two XB-29 when an engine fire developed. The port wing spar burned through and collapsed sending the huge bomber crashing into a meat packing plant a few miles south of Boeing Field. All eleven men aboard the plane and 18 in the plant were killed instantly."
