Nimrod development began in 1964 as a project to replace the elderly Avro Shackleton. Like many other successful maritime patrol aircraft, it was based on a civil airliner which had reached the end of its market life, in this case, the Comet 4. The Comet's turbojet engines were replaced with Rolls-Royce Spey turbofans (for better fuel efficiency, particularly at the low altitudes required for maritime patrol), and major changes made to the fuselage, including an internal weapons bay, an extended nose for radar, a new tail with ESM sensors mounted in a bulky fairing, and a MAD boom. In aggregate, the airframe changes turned the slim, graceful Comet into one of the ugliest aircraft ever made, but after a first flight in May 1967 the RAF ordered 46 Nimrod MR.1s, and the first example entered service in October 1969.
The Nimrod serves the RAF in two variants, the R.1P variant in a reconnaissance and electronic intelligence gathering capacity (officially, these were once coyly described as "radar calibration aircraft"), and the MR version in the maritime reconnaissance role. The R.1P variant is recognisable by the fact it doesn't have a MAD boom unlike the MR version.
Starting in 1975, 32 aircraft were upgraded to MR.2 standard, involving modernisation of the electronic suite and (as the MR.2P) provision for inflight refueling and additional ESM pods on the wingtips.
In the mid-1980s, the Nimrod's duties were expanded to include AEW - again, as a replacement for the Lancaster-derived, piston-engined Shackleton which, astonishingly, was still in service in that role. The Nimrod AEW.3 project was plagued by cost over-runs and electronic difficulties. Eventually, it was realised that the cost of developing a state-of-the-art all-British AEW system for a mere handful of aircraft was unsustainable, and the RAF received much cheaper and more effective mass-produced Boeing E-3 Sentries ("AWACS") instead.
Towards the end of the 20th century, BAe began planning to build a Nimrod replacement. The RAF considered bids from Lockheed with its P-3 Orion, Loral with rebuilt ex-US Navy Orions, and Dassault with the Atlantique 3, but in July 1997 awarded the contract to BAe with the substantially rebuilt Nimrod MRA.4. The MRA.4 is essentially a new aircraft, with current-generation BMW Rolls-Royce BR710 turbofan engines, a new, larger wing, and fully refurbished fuselage.
Development has taken longer than anticipated but the first of 18 MRA.4s are expected to enter service shortly. For the RAF, the jet-powered Nimrod offers greater speed and range than any turboprop, and enhanced ability to detect a threat without itself being detected because of the reduced noise and lower radar signature of the Nimrod's buried engines. New Bombardier Sentinel aircraft due for delivery from mid 2004 may take on some duties performed by the R.1P variant described above.