at RAF Wittering in May, No. 148 Squadron at RAF Marham in July, No. 7 Squadron at RAF Honington in November and finally No. 90 Squadron at RAF Honington in January 1957. Vulcan XA895 was allocated to No. 230 Operational Conversion Unit at RAF Waddington in January 1957, and Vulcan aircrew training commenced. The first Vulcan squadron, No. 83 Squadron, was formed at RAF Waddington in May 1957. It initially used aircraft borrowed from No. 230 Operational Conversion Unit until it received its first Vulcan, XA905, on 11 July 1957. It was followed by No. 101 Squadron, which was formed at RAF Finningley on 15 October 1957. A third Vulcan squadron, No. 617 Squadron, was formed on 1 May 1958 at RAF Scampton, the same base from which it carried out the Dambuster Raids in May 1943. No. 232 Operational Conversion Unit took delivery of its first Victor on 29 November 1957. The first operational Victor squadron was No. 10 Squadron RAF, which received its first Victor on 9 April 1958 and was formed on 15 April. This was followed by No. 15 Squadron, which was formed on 1 September 1958, and No. 57 Squadron, which was formed on 1 January 1959. The UK nuclear strike force became known officially as the V force or Main Force. V force assets at the end of 1958 were: Second generation The development of effective jet fighters and anti-aircraft missile defences promised to make the nuclear deterrent delivered from bombers flying at high altitudes increasingly ineffective. While making the V bombers fast enough to avoid them was problematic, improved engines offered the possibility of allowing them to fly higher. Since the Mikoyan-Gurevich MiG-19 fighters coming into service in the Soviet Union had a ceiling of , a V bomber could avoid them by flying at over . Two dozen of a new model of the Vulcan, the B.2, with Bristol Olympus 201 engines, a slightly larger wingspan and new electrical and electronic systems were ordered on 25 February 1956. The last 17 aircraft outstanding from the September 1954 order and 8 from the March 1955 order were switched to the B.2, making a total of 49 on order. Another 40 were ordered on 22 January 1958. A pre-production model, XH533, first flew on 19 August 1958, and in a trial on 4 March 1959 it reached . The wings and new engines also