Starting in 1959, Martin began to modify retired RB-57As with electronic countermeasures (ECM) equipment in the bomb bay. Redesignated "EB-57A", these aircraft were deployed with Defense Systems Evaluation Squadrons which played the role of aggressors to train the friendly air defense units in the art of electronic warfare. Subsequent bomber variants were also modified to fulfill this role. Although initially conducted by active duty Air Force units, the EB-57 mission eventually migrated to selected units of the Air National Guard. The ANG's EB-57s were replaced, in the 1980s, by the USAF's more advanced General Dynamics/Grumman EF-111A Raven operated by the active-duty USAF.