B-52
There are no current military aircraft with 6 engines.
Any bomber's use is ONLY for the military.
The Boeing B-47 Stratojet strategic bomber had six engines. It is no longer in service. Also, during WWII, the Germans made a powered glider called the Me-323. In the '70s, the US made three prototypes of the XB-70 Valkrye. Besides, the Boeing B-52 also appears to have 6 engines. Actually, it has an engine pod in place of a single engine per support pole. The engine pod has 2 engines each, so if you ever see a huge military aircraft in the US with 6 engines, then you must know that it's actually 12!
The B-47 Stratojet had 6 engines, but is no longer in service.
It has 8 engines, four on each side, two per boom.
A B-52 has 8 TF33-P-3 turbofan engines
The Russian Tupolev Tu-160 is the world's largest bomber.
The question of the cost of a military aircraft is hard to answer. The reason is the military will buy a production run that includes, support, technical documentation & drawings, training, and manuals, spare parts and spare engines. The Army does not usually go out and buy 1 airplane like we buy a car. The B-29 is an old airplane that is no longer made. So are you referring to the original cost of this bomber or the cost to buy a restored, flying bomber?
Good aerodynamics and powerful engines.
I assume you mean during World War 2. They made the Jeep, Tanks, Military Trucks, Half Tracks, Grenades, Landing Craft, Bomber Engines, B24 Liberator Bomber and possibly other war material.
Apart from the obvious increase in the number of engines, there is no real difference. The extra engines give increased thrust and thus additional "bomb" capacity. Depending on the role intended, a four engined bomber will be preferred as in a war, fuel is not a consideration.