To be clear, it's "Built almost entirely by women, men refused to fly it citing safety concerns [about the engines catching fire]." and not "...men refused to fly it citing safety concerns [because it was built by women]."

I think the logic was more like a ego thing, like it cannot be that they can do it and we not, as seen in many man first cultures.

Interestingly, "Flight of the Enola Gay" by Paul Tibbets does not mention this, though other narratives did.

The B-29 did have a major problem with engine fires, due to poor cooling of the engines and magnesium parts. An early test flight of it had such a fire, killing the 11 man crew. This problem was never fully resolved, and the B-29s were pushed into combat with this problem. Having an engine fire somewhere over the Pacific must have been utterly terrifying.

The B-29 also did not have hydraulic boost. It's a much bigger and heavier airplane than the B-17, which also did not have boost. The B-17, under emergency conditions, required a great deal of strength on the part of the pilots. My dad flew B-17s. He said that if 3 engines were out and only one outboard engine was running, it took all of a man's strength stepping on the rudder to keep the airplane straight. Even so, he could only keep it up for 10 minutes at a time, switching between pilot and copilot.

WW2 airplanes were designed with a man's strength in mind. The P-51, for example, required a great deal of strength to control it at high speeds and in emergency conditions.

Women pilots were used to ferry P-51s. Many were lost in crashes. My dad, who also flew P-51s, suspected that they ran into conditions where they weren't strong enough to control it.

The Night Witches flew fighter planes that were half the weight of the P-51, making them well suited for female pilots.

The advent of hydraulically boosted flight controls resolved this. Boeing lightened the "feel" forces on jet airliner controls to accommodate women pilots.