What navigational aid(s) would WWII bomber pilots have used while flying in instrument conditions?
what navigation aid(s) would these pilots have used while flying on
  instruments?


The short answer is the guy sitting behind them (aside from your standard 6 pack) 

On planes of this size and of this era most of the navigation and operation of navigation instruments was up to the flight navigator. They sat behind the pilot and sometimes had other instrumentation back there. In many cases they had a table and a decent map as well as some stop watches a pencil and a good straight edge. The pilots were responsible for things like heading hold, altitude hold, and speed hold. Although some planes even had aft placed throttles out of reach of the pilots. 


(source)

In areas of the world where NDB's and Other early radio nav ads were not really present pilotage and dead reckoning were often used. Keep in mind VOR's were not deployed until 1946.

Celestial navigation was also used however that is dependent on sky condition at the given time. 

This B-29 "FiFi" Navigators station shows a pretty sparse setup of instrumentation that appears to include some kind of ADF style read out. 



(Source)

This site provides a great overview of some of the nav equipment used at the time. 

You can find the full 1945 "Flight Through Instruments" Naval Aviator publication here.

According to this article the B-18 did fly with a navigator


  The crew, numbering three, constituted the pilot and flight navigator
  in the traditional cockpit position over the nose with the bombardier
  seated in the position at the nose proper.


Early versions were even used to train navigators, 


  Work began in earnest on a variant specifically for training military
  pilots, bombardiers and navigators.





  the "loop antenna"


The loop antenna is a part of early ADF systems and on occasion can still be found on some aircraft (see ADF loop below). 


(source)

The loop antenna is one of two components needed for an ADF system. It is the component that can detect direction. However it only has 180 degrees of precision and is ambiguous beyond that. In other words it does not know if the signal is from the front or the back. The ADF Sense antenna solves the ambiguity problem (if installed).