Even before the B-26 began to suffer from fatigue problems in early 1964 the USAF had made plans to upgrade the aircraft.  In October 1962 a B-26 was delivered to On Mark Engineering of Van Nuys, California for a major upgrade that involved almost rebuilding the entire aircraft.  The result was the B-26K which featured a completely re-manufactured fuselage, wing and tail, more powerful engines, wing-tip fuel tanks, eight wing pylons for ordnance and updated cockpit instrumentation and avionics.  The first of 40 aircraft (sporting new 1964 serial numbers) was delivered to the USAF in June 1964 but by then the B-26 had been withdrawn from Vietnam and the need for replacement aircraft did not appear to be a high priority as it had been replaced successfully by the A-1 Skyraider.  The 603rd ACS worked up the aircraft at England AFB, Louisiana and after two years of testing and training including small scale deployments to the Congo and Panama, it was decided to deploy the aircraft to Southeast Asia.  Initially eight aircraft were sent to Nakhon Phanom in June 1966 on a six-month combat evaluation under the code name Big Eagle where, as Detachment 1 of the 603rd ACS, they were attached to the 606th ACS.  The aircraft’s designation was changed from B-26K to A-26A due to the sensitivity of basing bomber aircraft in Thailand at that time.  So succesful was the A-26A in the night interdiction role over the Ho Chi Minh Trail that it was decided to keep the aircraft in Southeast Asia on a permanent basis.  The unit ceased to be a detachment of the 603rd on 21 December 1966 but was absorbed into the 606th ACS and was assigned to the 634th CSG.  On 15 September 1967 the 609th ACS was formed at Nakhon Phanom to operate the A-26A.  The aircraft’s main roles were night interdiction of the Trail in the Steel Tiger area of southern Laos and close air support and air strikes in the Barrel Roll area in the north of the country.  The aircraft was only intended as a stopgap until fixed-wing gunships became available, however, this did not happen until late in 1968.  The A-26A was usually known as Nimrod after its radio call sign.