Lockheed began designing the Neptune during the Second World War as a dedicated land-based maritime patrol and anti-submarine aircraft — a category distinct from the flying boats and converted bombers that had done much of that work earlier in the war. The design emphasised long range, a big internal weapons bay, good crew facilities for long patrols, and easy maintenance.

The prototype first flew in May 1945, too late for wartime service, but the timing proved ideal for the Cold War that followed. Designed from a clean sheet for the patrol mission rather than adapted from a bomber, the Neptune was efficient and effective in the role, and it entered US Navy service in 1947 as the standard land-based patrol aircraft — a position it would hold for many years.

The Neptune made its name almost immediately with a record-breaking endurance flight. In late September and early October 1946, a specially prepared P2V-1 named the “Truculent Turtle” took off from Perth in Western Australia and flew non-stop and without refuelling to Columbus, Ohio, in the United States — a distance of over 18,000 kilometres, covered in roughly fifty-five hours.

It set a world distance record that stood for many years, a dramatic demonstration of the new aircraft’s extraordinary range. For a maritime patrol aircraft, range and endurance were the whole game, and the Truculent Turtle proved beyond doubt that the Neptune had them. The record aircraft itself survives and is preserved at the National Naval Aviation Museum — a tangible link to one of the great long-distance flights of the propeller era.

The Neptune’s most distinctive feature was the powerplant of its later marks. The aircraft was designed around two large Wright R-3350 radial piston engines, which gave it the fuel efficiency needed for very long patrols. But as the aircraft grew heavier with equipment and weapons, more power was wanted for takeoff and for a fast dash to a target — so Lockheed added two Westinghouse turbojets in pods under the wings.

The result was a hybrid: on a normal patrol the Neptune cruised economically on its two piston engines, shutting the thirsty jets down; when it needed extra thrust — for a heavy takeoff, a rapid climb or a quick run in to attack — the crew lit the jets. Airmen summed it up as “two turning, two burning”, and the combination of whirling propellers and screaming jet pods made the piston-plus-jet Neptune one of the most recognisable aircraft of its era.

Finding a submarine in the vastness of the ocean is one of the hardest problems in warfare, and the Neptune carried a whole suite of sensors to do it. Search radar detected submarines and ships on the surface. A magnetic anomaly detector (MAD), housed in the long tail “stinger” that gives the Neptune its distinctive profile, could sense the tiny distortion a submerged steel submarine causes in the Earth’s magnetic field as the aircraft passed low overhead.

Most important for tracking a submerged boat were sonobuoys — expendable listening devices dropped into the sea that radioed back the sounds they heard, letting the crew localise a submarine underwater. Some versions also carried a powerful searchlight to illuminate a surfaced target at night for attack. Together these sensors let a single Neptune search, detect, localise and attack — the full anti-submarine sequence in one aircraft.

The Neptune entered service just as the Cold War was beginning, and it became the US Navy’s standard land-based patrol aircraft through the 1950s and into the 1960s. From bases around the world it flew the long, patient patrols of the Cold War at sea — watching for the growing Soviet submarine fleet, tracking shipping, and standing ready to hunt and attack if war came.

It also saw combat. During the Korean War, Neptunes flew maritime patrol, reconnaissance and mining missions, and the type was involved in some of the tense reconnaissance operations of the early Cold War, when patrol aircraft probing the edges of Soviet and Chinese airspace were sometimes attacked. For the US Navy the Neptune was the everyday workhorse of the ocean watch for the better part of two decades.

The Neptune found unexpected new roles in the Vietnam War. Specially modified aircraft, the OP-2E, were used to drop the air-delivered electronic sensors of the “Igloo White” system along the Ho Chi Minh Trail — seismic and acoustic sensors that detected trucks and troops moving through the jungle and relayed the data to be acted on. It was dangerous, low-level work over heavily defended territory.

Other Neptunes were converted into heavily armed gunships, the AP-2H, carrying guns, sensors and ordnance for night interdiction against the same supply routes. These were far from the aircraft’s designed maritime-patrol role, but they show how adaptable the big, capable airframe was — and how the Neptune kept finding work well beyond hunting submarines at sea.

At the heart of the Neptune were two Wright R-3350 radial engines — the same powerful, complex eighteen-cylinder family that powered the B-29 Superfortress and the Lockheed Constellation. In its turbo-compound form the R-3350 recovered energy from the exhaust to boost power and efficiency, giving the Neptune the fuel economy it needed for very long-range patrol.

Range and endurance were the Neptune’s defining strengths, as the Truculent Turtle flight had shown. A patrol aircraft’s value lies in how far out it can reach and how long it can stay there, and the R-3350-powered Neptune could cover enormous areas of ocean. The later addition of jet pods did not change that basic economy — the jets were for bursts of power, while the efficient radials still did the long, patient work of the patrol.

By the late 1950s the US Navy wanted a more capable land-based patrol aircraft, with more room for the ever-growing suite of anti-submarine sensors and processing equipment, more endurance, and the smoothness and reliability of turboprop power. Lockheed’s answer was the P-3 Orion, developed from its Electra airliner, which began replacing the Neptune in US Navy squadrons from the early 1960s.

The Orion was bigger, longer-ranged and more advanced, and it went on to serve for decades as one of the most successful maritime patrol aircraft ever built. But the Neptune was not simply discarded: it continued in US Navy reserve and specialised roles for years, and it soldiered on with many foreign air arms long after the Orion had taken over the front-line American mission. The two Lockheed patrol aircraft are directly linked — the Orion carried forward the land-based patrol tradition the Neptune had established.

Japan took the Neptune furthest of any foreign operator. The Japan Maritime Self-Defense Force operated American-built Neptunes, and then Kawasaki developed a substantially modified version under licence: the P-2J. It replaced the Neptune’s piston engines with turboprops (and updated jet pods), stretched the fuselage, and fitted more modern equipment, tailoring the design to Japanese requirements.

The P-2J was built in Japan in useful numbers and served the JMSDF as a maritime patrol and anti-submarine aircraft for many years. It is a notable example of a licensed design being genuinely developed rather than merely copied — a distinctly Japanese evolution of the Neptune that kept the basic concept in front-line service well into the later Cold War, before it too was replaced by the P-3.

The Neptune was produced in a long line of variants, from the P2V-1 of 1947 through to the definitive P2V-7. Along the way it gained the underwing jet pods, more powerful radials, updated radar and sensors, and a steadily heavier anti-submarine fit. Specialised versions included electronic-reconnaissance aircraft, the Vietnam-era OP-2E sensor-droppers and AP-2H gunships, and ski- and float-equipped machines for polar work.

In 1962 the United States rationalised its aircraft designation system, and the Neptune’s Navy designations changed: the P2V-5 became the P-2E, the P2V-7 became the P-2H, and so on. This is why the same aircraft is referred to both as, say, the P2V-7 and the P-2H depending on the date of the source. The airframe did not change — only the label — but it can cause confusion when reading about the type.

As with most Cold War military aircraft, there is no single reliable, transparent public figure for the Neptune’s unit cost, cost per flight hour or total programme cost. It was procured over many years, in many variants, for many customers, through government contracts and military-assistance programmes rather than at a transparent list price, and consistent lifetime figures are not available. Any single figure quoted for it should be treated with caution.

In general terms, a large four-engine (two piston, two jet) patrol aircraft with a big crew and an elaborate sensor fit was an expensive and demanding aircraft to buy and operate — but far cheaper than the fleet of ships that would otherwise be needed to cover the same ocean areas, which is exactly why so many navies invested in land-based patrol aircraft. We list the cost fields as “not reliably recorded” rather than invent numbers.

The Neptune was one of the most widely exported patrol aircraft of its era. Beyond the US Navy it served the maritime and air arms of Australia, Argentina, Brazil, Canada, France, Japan and the Netherlands, and Britain’s RAF Coastal Command flew leased Neptunes for a few years in the 1950s as an interim aircraft. Several other nations operated it as well.

These aircraft did the same essential job all over the world — patrolling coasts and sea lanes, hunting submarines and gathering intelligence — and often kept flying long after the US Navy had moved on to the P-3. Argentina’s Neptunes, for instance, were used for maritime reconnaissance during the 1982 Falklands conflict, near the very end of the type’s front-line career, before being retired. The breadth of the Neptune’s service is a measure of how well it filled a role that every maritime nation needed filled.

The Neptune had a long and productive third life after military service. Retired aircraft, with their strong airframes, long endurance and big load-carrying ability, made excellent aerial firefighting air tankers, and converted Neptunes flew for decades dropping fire retardant on wildfires across the western United States, continuing into the twenty-first century before finally being replaced by more modern tankers.

Thanks to that long working life and reasonable production numbers, a good number of Neptunes survive. Museum examples are preserved around the world, including the record-setting Truculent Turtle at the National Naval Aviation Museum in Pensacola, and a few remained flyable in the firefighting fleet until relatively recently. For an aircraft first flown in 1945, the Neptune’s span of active service — military patrol, reconnaissance, and firefighting — stretching into the 2010s is a remarkable testament to a sound design.