The United States has a lengthy coastline on both its east and west coasts. In the 1930s it also had the territories of Guam (which it still holds) and the Philippines, both of which were acquired during the Spanish-American War of 1898. Also the winds of war were blowing off both coasts.

There was a need to protect the US's sea routes from both surface and underwater threats, as well as an ability to disrupt the enemy's shipping. A further requirement was the ability to supply front line forces by air, ideally without needing to rely on established runways. This required long range aircraft; specifically, flying boats. In 1933 the US Navy put out contracts for a modern plane to do that.

They selected one of three competing designs; the Consolidated Aircraft company's PBY Catalina. (PB = Patrol Bomber, Y was the designation for Consolidated (since "C" had been taken by Curtis Aviation), and Catalina refers to Catalina Island, not so far from Consolidated's then headquarters in San Diego California.) The model made its first flight on 21 March 1935.

The Catalina's design followed function. The bottom of the hull is of course shaped like a boat. The original versions had no wheels at all; they landed only on water. Later models (the ones with "A" after their name, like this one) were amphibians; they had landing gear as well, allowing them to land on airfields as well as water. (On water the nose gear was fully retracted, and the wheels remained inside the recesses that you can see here.)

The wing is mounted high, sitting on top of a pylon, and supported by external struts as you see here. This allowed it to have a long wing (104' or 32m, as long or longer than many 4 engine bombers like the Lancaster and the B-17) with a high aspect ratio (roughly length to width ratio) which gave it a lot of lift. Retractable floats, which became part of the wing's surface, could be found out on the wing tips.

The high wings also allowed the twin Pratt & Whitney Twin Wasp 14 cylinder radial engines (1200 hp / 890 kW) to be mounted high, reducing the risk of them being damaged by water take-offs and landings.

The cruise speed was 109 knots (201 km/h, 125 mph) with a maximum 170 kn. The range was 4,060 km (2,520 miles), with a ceiling of 15,800'. It normally carried a crew of 8.

The Catalina was also in service in Australia with the Royal Australian Air Force (RAAF), which had similar issues to deal with. (As far as I can tell all Australian ones were flying boats rather than amphibians like this one.) The Catalina also saw some wartime quasi-commercial service with Qantas. Qantas offered a weekly non-stop service between Perth and Colombo (then Ceylon, now Sri Lanka) to reestablish an air link between Australia and England after the fall of Singapore. The "Double Sunrise" flights were the longest non stop service to date. They took 27 to 33 hours covering 6,500km, crossing Japanese held territory at night and seeing the sun rise twice. Wait, didn't I say that the range was 4,060km? Yes, they stripped out all non-essential equipment (including guns and heating equipment), added fuel tanks and limited the passengers to 3 and 69kg (152lb) of essential mail.

By the end of the war technological changes had reduced the effectiveness of the Patrol Bomber and anti-submarine aspects of the Catalina's work, but it was still useful in roles like search and rescue. It was retired from the US Navy Reserve at the start of 1957, but was still serving with the Brazilian military until 1982.

It also had non-military uses. After the war they were used to fly passengers to tropical locations, including from the former seaplane base at Rose Bay in Sydney. A lot of the services were operated by Qantas' rival, Ansett.

The Catalina was also sometimes used as a firefighting water bomber.

There are still a few flying as historic aircraft, but I'm not sure whether any of those are still flying operationally.

Addendum: My personal opinion is that Facebook is a sociopathic organisation that steals other peoples' intellectual property (including the images in this gallery) despite being explicitly denied permission to do so.