The Cessna A-37 Dragonfly was a modified version of the T-37 primary trainer that had been in service with the USAF since 1957.  The Air Force had been interested in a counter-insurgency version of the T-37 since 1963 but it was the higher than anticipated attrition rate of the A-1E in Southeast Asia that finally decided the issue.  The airframe of the basic T-37 was strengthened to withstand the higher stresses and loads encountered in combat and the T-37’s J69 engines were replaced by J85s that had more than twice the thrust.  The wing was rebuilt to take eight hardpoints for carrying up to 4,855lbs of ordnance and a GAU-2B/A Minigun was mounted in the nose.  The aircraft was fitted with wingtip fuel tanks, although even with these its range was only some 390 miles.  The first 25 A-37As were assigned to the 604th ACS at England AFB, Louisiana, which moved to Bien Hoa in August 1967.  The Squadron was to undertake a four-month combat evaluation programme named Combat Dragon.  Flying at Bien Hoa commenced on 15 August and within three weeks the daily sortie rate had increased from 12 to 60 per day.   In October a detachment of aircraft was sent to Pleiku to participate in Tiger Hound operations in Laos.  The evaluation was completed in mid-December with more than 4,300 sorties flown for the loss of only two aircraft.  As a result of the evaluation a new version, designated the A-37B, was put into production.  Modifications to the aircraft included the installation of a flight-refuelling probe in the nose to allow increased endurance.  After the Combat Dragon trial was completed the 604th continued to operate the A-37 at Bien Hoa and had flown a total of 10,000 sorties by May 1968.  A large number of A-37Bs were later supplied to the VNAF, which eventually operated 10 squadrons of the type.  The USAF only equipped three squadrons with the A-37 in Southeast Asia.