sentences:  Flight testing of the equipment in two Boeing B-29 Superfortresses began in May with such success that in June the decision was made to retrofit all of its first-line B-50 bombers with aerial refueling equipment.The world's first two dedicated air refueling units, the 43rd and 509th Air Refueling Squadrons, were constituted on June 30 and became operational in January 1949.With the development in 1948 of a "flying boom" delivery system, followed in 1949 by the "probe-and-drogue" system, in-flight refueling of fighter aircraft using single-point receiving equipment also became practical and USAF committed itself to the concept for the bulk of its future combat aircraft.