During the late war years a lot of effort was put in the development of the new jet engines. People envisaged the use of these new engines for all types of aircraft, including passenger planes.
However by 1945 it became clear that the jet wouldn't become a reliable propulsion source anytime soon. Aircraft manufacturers quickly had to convert their plans to the use of piston engines for their future designs.
The Boeing company decided to use the B-29 wing and engines for its new airliner. The wings were extended to add an extra engine on each side. the fuselage accomodated 100 passengers. The aircraft was an immediate success and over 500 were built in various sub types. The USAF used the Boeing 505 as the C-136 transport.
A bomber variant with a new fuselage was proposed but not proceeded with.
The Boeing 505 was the first of a series of very succesful Boeing airlines comprising the 525, the 535 and the 545 Jumboliner.