When the US Air Force saw that the Boeing RB-50 and North American RB-45 reconnaissance aircraft were becoming vulnerable to the latest Soviet fighters, it welcomed the opportunity offered by the forthcoming B-66, and the first of five B-66As flew in June 1954, well before the B-66B bomber model. From the later 1950s the B-66 was replaced by the Republic F-105 Thunderchief, but the reconnaissance variants remained in full service. Next to appear was the RB-66B with the RB-66As four camera installation but more power and a removable in-flight refueling probe. The first of 145 aircraft flew in March 1955, and the model remained in first line service up to 1966. The RB-66C was an electric reconnaissance and countermeasures model with four operators in a pressurized bomb bay compartment plus the option of a chaff dispenser in place of the tail barbette. The first flew in October 1955 and the 36 aircraft were very important in the first stages of the Vietnam War.