Turbofan engines, which add thrust by compressing more air than will go through the burners and turbine, provided more efficient thrust by increasing the mass flow for the same amount of fuel. The new Pratt & Whitney TF30 and the variable geometry wing were to be combined in the F-111 for far greater range than previously possible in a tactical fighter.
The turbofan had been introduced with commercial airliners so it was considered to be low risk. Unfortunately, in those pre-Concorde days, airliners did not need afterburner or fly at Mach 2. General Dynamics and its subcontractor, Grumman, quickly discovered how intolerant the turbofan was of flow conditions that the fully developed axial-flow jet engines took in stride.
There were three major inlet configurations on the seven F-111Bs that flew, two of which had two different inlets during the flight test program, and Grumman wasn't even the lead for inlet development. It just completed its F-111Bs with the most promising inlet that General Dynamics had defined at the time and retrofitted the carrier-trials prototype and the first pre-production aircraft with the subsequent one.
For a complete and more-balanced history on the F-111B, order my monograph on the F-111B here
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