Multi Engined bombers..

[ElAurens]

From Wiki...

Boeing submitted its Model 345 on 11 May 1940,[5] in competition with designs from Consolidated Aircraft (the Model 33, later to become the B-32),[6] Lockheed (the Lockheed XB-30),[7] and Douglas (the Douglas XB-31).[8] Douglas and Lockheed soon abandoned work on their projects, but Boeing received an order for two flying prototypes, given the designation XB-29, and an airframe for static testing on 24 August 1940, with the order being revised to add a third flying aircraft on 14 December. Consolidated continued to work on its Model 33 as it was seen by the Air Corps as a backup in case of problems with Boeing's design.[9] An initial production order for 14 service test aircraft and 250 production bombers was placed in May 1941,[10] this being increased to 500 aircraft in January 1942

http://en.wikipedia.org/wiki/Boeing_B-29_Superfortress