The injuries ranged from critical to minor, U.S. Marine Corps Forces Pacific spokesman Capt. Alex Lim said.
The tilt-rotor MV-22 Osprey, which can take off and land like a helicopter but flies like an airplane, had a "hard-landing mishap" at about 11:40 a.m., the 15th Marine Expeditionary Unit said in a statement.
The cause of the crash was under investigation, Lim said.
Twenty-two people were aboard the aircraft, including 21 Marines and one Navy corpsman assigned to the unit, spokesman Capt. Brian Block said in an email.
The 15th Marine Expeditionary Unit is based at Camp Pendleton in California and is in Hawaii for about a week for training. The Osprey was being used for training at Bellows Air Force Station on Oahu at the time of the hard landing.
Kimberly Hynd said she was hiking the popular Lanikai Pillbox Trail and could see three Osprey aircraft performing maneuvers from her vantage point in the hills above Bellows. She noticed them kicking up dirt but then saw smoke and fire. Hynd, who estimated she was 2 to 3 miles away, didn't hear the sound of a large crash.
"It looked like they were doing some sort of maneuver or formation -- and so I was taking pictures of it because usually you can't see them that close up," Hynd said.
Donald Gahit said he saw smoke rising in the air from Bellows when he looked outside his house after hearing sirens pass by.
"At first I thought it was clouds, but it was moving fast and it was pretty dark," the Waimanalo resident said.
Ospreys may be equipped with radar, lasers and a missile defense system. Each can carry 24 Marines into combat.
Built by Boeing Co. and Bell, a unit of Textron Inc., the Osprey program was nearly scrapped after a history of mechanical failures and two test crashes that killed 23 Marines in 2000.
The aircraft have since been deployed to Iraq and Afghanistan. Some Osprey are also helping with earthquake relief efforts in Nepal.