the building of eng group was not a mistake per say
more the way it was done in the beginning!
for the B-29 at a cost of around $3.5 billions i mean this was like around 40 billions in todays dollars this is as high as the A bomb project
and was not ready till end of 1944
note : there was some court cases won by the government for this B-29
One must question whether these aircraft were cost effective. Each aircraft was hideously expensive, at over half a million dollars,
or five times as much as a Lancaster.
Each required over thirteen tons of aluminum, half a ton of copper, 600,000 rivets, nine and a half miles (15 km) of wiring,
and two miles (3 km) of tubing. In addition to the regular crew, each aircraft required 74 relief and ground crew.
Great things were expected from the aircraft, but they were judged a failure in their original design role as conventional
high-altitude daylight strategic bombers. They were more successful as low-altitude nighttime fire bombers,
but only because Japanese air defenses were pitifully inadequate. (Losses to interceptors averaged just 0.24% of sorties.)
Their employment as the world's first nuclear bombers, albeit in the Silverplate variation, has probably muted what
would otherwise be justified criticism of the B-29 program. No other aircraft could have delivered the nuclear bombs to Japan.
but still US at beginning of war did not have any large military or military manufacturing system
So it was more like a rush to get everything done at same time and what ever cost it was without know how !
problem is that no companies or even government departments had the knowledge or experience to do that to such a scale meaning many errors happened
but i guess this is a learning process and history of WWII