Here is an info about Model 279-3JF;
To investigate the benefit in increased payload, the 279-3 and
the jet flapped 279-3JF were modeled on NASA's "Aircraft Synthesis
(ACSYNT) computer code and flown on a 250 ft. takeoff distance interdiction
mission. The increase in payload weight that the 279-3JF could
carry was converted into fuel in one case, and in another, converted
to bomb load. When the fuel was increased, the 279-3JF penetrated into
enemy territory almost four times the distance of the 279-3, and therefore
increased mission capability. When the bomb load was increased,
the 279-3JF carried 14 bombs the same distance the 279-3 carried four.
This increase in mission performance and improvements in turning rates
was realized with only a small penalty in increased empty weight.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19860021250_1986021250.pdf