Since we don't need a control tower for instrument landings I was wondering what else that is thought to be needed could be skipped. I know some small general aviation is done on grass runways and I was wondering what is the biggest airplane that does not need a paved runway? Examples of manufacturer-approved usage or regular (e.g. daily or weekly) usage are preferred.
Military transports are designed to be operated from unpaved runways- by extension, any of these converted to civilian use can operate from unpaved runways.
Multi-leg landing gears equipped with 24 wheels allow to operate the aircraft on unpaved runways ...
The H-4 Hercules (Spruce Goose) should easily win this contest. That plane was ridiculously huge and, in fact, couldn't use a runway, rather it was a "flying boat" that took off from the the water. So I'd say it's the one...
Granted, if you mean a grass or gravel strip aeroalias is probably correct. The only other major planes that I know were designed to land on gravel were the early 737 and the 727. They both had special landing gear guards that keeps the front gear from kicking gravel into the engines and both have specific instructions for preparing the landing gear for touch down on gravel.
Here's the guard, if you were curious, on a 737:
Source https://www.flickr.com/photos/capnmikesphotos/14891693505, Author: Mike Pearson
It's not quite as big as the Antonov An-124, but honorable mention would have to go to the Lockheed LC-130. It's designed to resupply scientific and military operations in polar regions, and so it has a dual wheel-and-ski landing gear setup that allows it to take off from and land on snow and ice.
These oldies seem worth mentioning...
If you loosen your definition of "airplane" to "the longest class of flying machine and the largest airship by envelope volume" you could think of a Hindenburg-class airship that was in regular commercial service.
Hindenburg in green compared to
Blue: The Pentagon building,
Pink: Queen Mary 2, ocean liner
Yellow: USS Enterprise, supercarrier
Dark blue: Yamato, WWII Japanese warship
Grey: Empire State Building
Red: Mont, a supertanker
Or the Spruce Goose (* Not acutal Spruce). (Just noticed it mentioned in another answer, after posting this)
Originally designated HK-1 for the first aircraft built by Hughes-Kaiser, the giant was re-designated H-4 when Henry Kaiser withdrew from the project in 1944. Nevertheless, the press insisted on calling it the “Spruce Goose” despite the fact that the plane is made almost entirely of birch.
Apparently it is "the largest flying boat ever built and has the largest wingspan of any aircraft in history" at 320 ft 11 in (97.54 m). Not strictly a "regularly used" airplane, but
Hughes retained a full crew to maintain the mammoth plane in a climate-controlled hangar up until his death in 1976.
Going off on another direction if ice counts as "unpaved", a Boeing 757 was landed in Antarctica in November 2015, the first commercial airliner to do that.
Source is from the link above.
Another option might be the Soviet Ekranoplan: these are aircraft that use the ground effect to fly just off a surface of water.
Sadly neither of these are flying any longer.