Explorer II was a crewed U.S. high-altitude balloon that was launched on November 11, 1935, and reached a record altitude of 22,066 m (72,395 ft).

Launched at 8:00 am from the Stratobowl in South Dakota, the helium balloon carried a two-man crew consisting of U.S. Army Air Corps Captains Albert W. Stevens and Orvil A. Anderson inside a sealed, spherical cabin.

[2] In January 1934, the National Geographic Society (NGS) and the U.S. Army Air Corps decided to collaborate on a program to build and launch a manned balloon to the then record altitude of 24 km (15 mi).

But news of the fatal crash of a Russian stratospheric attempt in 1934 left President Gilbert H. Grosvenor of the NGS sobered by the risk.

[5] A review of the crash by the National Bureau of Standards[5] (NBS) was held between July and September, revealing that the balloon had not opened symmetrically during the ascent, causing stresses that led to the fabric tears.

A month-long delay prior to launch had allowed the rubberized cotton to stick together, which created the uneven expansion.

The lower lifting efficiency of helium gas meant that a larger balloon would be needed,[5] so Goodyear-Zeppelin[3] increased the volume to 100,000 m3 (3,700,000 cu ft).

Dow Chemical Company assembled a larger, lighter gondola[6] made of "Dowmetal"[3]—a magnesium-aluminium alloy[7]— that would carry a two-man crew with a reduced amount of scientific instrumentation.

The meteorological requirements for the launch was for clear skies—with no precipitation—lasting for the duration of the flight, as well as surface wind speeds that were not to exceed 23 km/h (14 mph).

The task of inflating the balloon with helium from the 1,685 steel cylinders[3] took eight hours,[10] during which the team needed to repair a 5.2 m (17 ft) long tear that formed in the fabric.

[3]) Lift-off occurred at precisely 8:00 am with the release of 34 kg (75 lb) of ballast made of fine lead shot.

[7] Explorer II included communications equipment, and constant radio contact was maintained throughout the flight with the signal being broadcast across the U.S. and in Europe.

The precautions proved unnecessary as the balloon landed gently in an open field near the town of White Lake, South Dakota at 4:13 pm.

[11] The balloon used for the Explorer II expedition was cut up into a million strips and distributed as commemorative bookmarks among the NGS members who supported the mission.