The Avro Lancaster II is a four-engine variable-sweep wing strategic bomber used by the Royal Air Force (RAF) and Royal Canadian Air Force (RCAF). It was first envisioned in the 1970s as a supersonic replacement for the aging V-Bomber forces of the British Empire and Canada, but also developed into a low level subsonic infiltrator with long range. Designed by A. V. Roe & Co. (Avro), the Lancaster II entered service in 1986 with both countries as a nuclear bomber. Throughout its development, there were several attempts to cancel it as the theory of strategic balance changed from airborne standoff to missile overkill in the 1970s. The signing of the Nuclear Delivery Systems Treaty in 1980, which banned any nuclear weapon delivery system that could not be recalled after launch, proved to be the Lancaster II's salvation.