Military transports are designed to be operated from unpaved runways- by extension, any of these converted to civilian use can operate from unpaved runways.
For example, the Antonov Airlines operates a number of Antonov An-124 'Ruslan' , which can and do operate from unpaved runways regularly.
The Volga-Dnepr group specifically states: Multi-leg landing gears equipped with 24 wheels allow to operate the aircraft on unpaved runways ...
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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
aviation.stackexchange.com/users/511/Jay Carr
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.
(Image source: Wikimedia Commons)
aviation.stackexchange.com/users/13297/Michael Seifert
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.
They landed in fields
They're more comparable in size to buildings or very large boats, rather than other airplanes 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 Compared to other airplanes (Hindenburg in orange): 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.
aviation.stackexchange.com/users/4506/Xen2050
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.
aviation.stackexchange.com/users/12220/SMS von der Tann
Another option might be the Soviet Ekranoplan: these are aircraft that use the ground effect to fly just off a surface of water.
For example we might have the Lun-class (73m): or the experimental Caspian Sea Monster (92m): Sadly neither of these are flying any longer.
(source:
Wikipedia Ground effect vehicle and Lun-class ekranoplan )
aviation.stackexchange.com/users/14510/user1908704
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.
What is the biggest airplane that does not need a paved runway?
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<aircraft-design><runways><aircraft-limitations>
aviation.stackexchange.com/questions/26906
aviation.stackexchange.com/users/2910/hildred
Military transports are designed to be operated from unpaved runways- by extension, any of these converted to civilian use can operate from unpaved runways.  Examples of these would be Antonov An-124, Ekranoplan and the H-4 Hercules.  Military transport aircraft that can be adjusted for civilian use, like the Antonov An-124 'Ruslan' planes or the Soviet Ekranoplan that can take off a surface  of water, are considered the largest. However, if we broaden the definition of "airplane", then Hindenburg-class airships like the H-4 Hercules (Spruce Goose) would definitely come first.
Military transport aircraft that can be adjusted for civilian use, like the Antonov An-124 'Ruslan' planes or the Soviet Ekranoplan that can take off a surface  of water, are considered the largest. If we broaden the definition of "airplane", then Hindenburg-class airships like the H-4 Hercules (Spruce Goose) would definitely come first.