The F-105D was originally intended for the nuclear strike role, with the primary armament being a "special store" (a euphemism for a nuclear weapon) housed in the internal weapons bay. This weapon was usually a Mk 28 or a Mk 43. However, an Mk 61 could be carried underneath the left or right inboard underwing pylon and a Mk 57 or a Mk 61 could be carried underneath the centerline pylon. However, as nuclear war became less and less likely, the nuclear weapon carried in the internal weapons bay was usually replaced by a 390-gallon internal fuel tank, the offensive load being carried on four underwing pylons and on a pylon mounted underneath the fuselage on the centerline. From the F-105D-20-RE production block onward, the Thunderchiefs had increased emphasis on conventional weapons delivery, with the capability of carrying a wide-ranging combination of external stores underneath the wings and fuselage. Many of the earlier production block F-105Ds were eventually upgraded to this standard.