The B-36 was originally designed in 1942, when it looked like the British might be overrun, and the USA would be left to fight on two fronts alone. The original spec was for a bomber that could carry a 10,000 pound bomb load 10,000 miles and operate from a 10,000 foot runway.
There was no computer modelling in those days, and the engineers could only make an informed guess about some of the design figures, such as the required power. The design included the six 27-cylinder reciprocating engines. (That's 324 spark plugs!)
The XB-36 flew in 1947, by which time the focus had turned to the Cold War requirements. Atomic bombs were large, heavy, and cumbersome, but the B-36 could handle them. The B-36A continued to fly with just the six piston engines, and the turbojets were added for the B-36B and all later models.
The turbojets burned jet fuel from dedicated fuel tanks; a turbine engine will not run on gasoline. The jets were used for takeoffs and for high-speed bombing runs.
With all engines running, the B-36 could make 432 mph in level flight.
As mentioned by one answerer, the jets worked out well and became the sole power source for the next generation of bombers, some of which are still flying. (The only replacement for a B-52 is another B-52.)
There are a couple of B-36s on display at museums, notably the SAC museum in Nebraska and the USAF museum in Dayton, OH. They are worth a trip to see.
For some human interest and some film of B-36s in flight, pick up the old Jimmy Stewart movie "Strategic Air Command."
Also, try an internet search on "B-36" and one on "XC-99." The XC-99 was a one-off cargo version of the B-36, and an amazing thing to see also. It still exists, though it is in pieces, and one hopes it will be restored and placed on display.
retired airline pilot
old B-36 fan.