By the late 1990s the US Navy’s P-3 Orions, designed in the 1950s, were wearing out. A first attempt to replace the Orion with a new-build version of the same aircraft was abandoned, and the Navy opened the Multimission Maritime Aircraft competition. Lockheed Martin offered a modernised Orion; Boeing offered a militarised 737. On 14 May 2004 Boeing won, and in June the Navy awarded a development contract worth more than US$3.8 billion.

The P-8 is built in an unusual way: Boeing’s commercial plant in Renton assembles a strengthened 737 airframe on the same line as airliners, and the military systems are installed afterwards. The basic design is a 737-800ERX with the wings of the 737-900, raked wingtips instead of the airliner’s blended winglets, a bomb bay behind the wing, six underwing and fuselage hardpoints and an air-to-air refuelling receptacle above the cockpit. The engines are CFM56-7s fitted with larger generators to power the mission systems.

India became the first export customer in January 2009, ordering eight P-8I Neptune aircraft with a magnetic anomaly detector and Indian-designed data links; four more followed in 2016 and further aircraft were approved later. The first arrived on 15 May 2013 and they serve with INAS 312 at INS Rajali, Arakkonam. Australia’s first P-8 arrived in 2016; the RAAF flies them from Edinburgh, South Australia, with Nos 11, 12 and 292 Squadrons under No. 92 Wing.

The United Kingdom, which had scrapped its Nimrod MRA4 in 2010, bought nine P-8s as the Poseidon MRA1. The first, named Pride of Moray, reached RAF Lossiemouth in October 2020, restoring British long-range maritime patrol after a gap of a decade. Norway flies five from Evenes in the Arctic north, New Zealand received four from December 2022 to replace its P-3K2 Orions, and South Korea and Germany have followed. Canada announced its choice of the P-8 in November 2023.

The US Navy gives the P-8A a crew of nine: two pilots and seven mission crew, including tactical coordinators and acoustic and electronic warfare operators; the RAAF lists eight. Unlike the Orion, whose crew needed an observer at the window, the P-8 crew works at a row of identical multifunction consoles along the left side of the cabin, any of which can be set to any task. Rotary sonobuoy launchers in the rear fuselage can be reloaded in flight.

The aircraft was designed for 25 years and 25,000 flight hours, according to Boeing, and receives regular capability increments: better acoustic processing, upgraded radar modes, wideband satellite communications and integration of new weapons and sensors. The P-8I adds the magnetic anomaly detector in a tail boom and a second radar at the rear, and some US aircraft carry an underbelly surveillance radar pod for overland and littoral work.

• P-8A Poseidon: US Navy baseline, also bought by Australia, Norway, New Zealand, South Korea, Germany and Canada.
• Poseidon MRA1: RAF designation for its nine P-8As at Lossiemouth.
• P-8I Neptune: Indian Navy version with magnetic anomaly detector, aft radar and Indian data links.
• Increment upgrades: successive US software and hardware packages adding sensors, communications and weapons such as the high-altitude torpedo kit.

One of the selling points of a 737-based patrol aircraft is commonality with the world’s largest airliner fleet: engines, spare parts, training and maintenance practices are widely available. That should make the P-8 cheaper to run per hour than an ageing four-engine turboprop or a bespoke design. GlobalSecurity, citing older Navy data, put the total life-cycle cost of the US programme at about US$44 billion in 2004 dollars.

Against that, the mission systems and sonobuoys are expensive, and maritime patrol is low-level work over salt water. No single authoritative public cost per flight hour was found; the estimate given is an order of magnitude only.