The F-106 was the follow on to the F-102 Delta Dagger. Originally designated F-102B the F-106 appellation was gained as the changes became more extensive. The Darts fuselage was redesigned to take the more powerful J75 turbojet and variable inlets were fitted. These modifications raised the Darts top speed from the Mach 1.25 of the Dagger to over Mach 2. Other changes included an upgraded Hughes MA-1 electronic guidance and fire control system, new undercarriage and the ability to employ the AIR-2 Genie unguided nuclear tipped ATA missile. An upgrade in 1973 included the provision of an M-61 20mm cannon in a semi retractable mounting in the weapons bay. First deliveries to Air Defence Command were in 1959 and the last examples were withdrawn from service in 1988.