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. The crew landed safely near White Lake, South Dakota, at 4:13 pm and both were acclaimed as national heroes. Scientific instruments carried on the gondola returned useful information about the stratosphere. The mission was funded by the membership of the National Geographic Society.

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). This vehicle would be capable of carrying a crew of three in an airtight capsule, along with a laboratory of instruments. The hydrogen balloon, named Explorer, was completed by July at a cost of around $60,000; equivalent to $1,410,299 in 2024 currency.

The balloon was launched from a canyon in the Black Hills of South Dakota—dubbed the Stratobowl—on July 28, 1934 and reached a near-record altitude of 18,475 m (60,613 ft) before tears in the fabric led the crew to begin reducing their altitude. A rupture in the balloon resulted in a precipitous descent, followed by a spark that caused the hydrogen to ignite and destroy what was left of the balloon, leaving the capsule to plummet toward the ground at terminal velocity. The crew just managed to escape using their parachutes, with the last man bailing out at 500 feet (150 m) above the ground. Their capsule was almost completely destroyed upon impact.

The crash resulted in a national embarrassment, leading Captain Albert W. Stevens, the scientific observer on board the Explorer balloon, to lobby for another attempt with an improved balloon to be named Explorer II. 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. A review of the crash by the National Bureau of Standards (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 hydrogen explosion followed when the gas in the bag mixed with the oxygen in the atmosphere.

Despite the concerns, in 1935 the NGS and Army Air Corps decided to make another attempt. To eliminate the hazard of the hydrogen lifting gas, it was decided instead to use helium—to which the United States had a monopoly. The lower lifting efficiency of helium gas meant that a larger balloon would be needed, so Goodyear-Zeppelin increased the volume to 100,000 m³ (3,700,000 cu ft). Dow Chemical Company assembled a larger, lighter gondola made of "Dowmetal"—a magnesium-aluminium alloy—that would carry a two-man crew with a reduced amount of scientific instrumentation. The cabin was 2.7 m (9 ft) in diameter with a mass of 290 kg (640 lb) and could transport a 680 kg (1,500 lb) payload. It was manufactured from a single, large plate that was cut up into form that could be reshaped and welded into a sphere. In order to make crew escape easier, the portholes were made wider than on the Explorer I. The atmosphere in the interior of the capsule was supplied from liquid air instead of liquid oxygen in order to reduce the fire hazard. The modified balloon was ready by the spring of 1935 and the first launch occurred July 10, 1935. Unfortunately, this too proved a failure with the balloon rupturing at liftoff.

Following a review by the NBS, the balloon was prepared for another attempt after Goodyear strengthened the material. Examination of climatological data for the Stratobowl collected over the previous fifteen years was examined, and it was determined that the month of October typically had periods of good weather that would last sufficiently long for a flight attempt. A team of meteorologists was assembled at the Stratobowl in early September and they proceeded to put together a weather station. 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).

With a cold front approaching, on the night of November 10, 1935, the balloon was prepared for launch. The temperature dropped to −14 °C (6 °F) overnight, so the 10,762.4 m² (115,845 sq ft) of fabric was kept warm and pliable through the use of stoves. The task of inflating the balloon with helium from the 1,685 steel cylinders took eight hours, during which the team needed to repair a 5.2 m (17 ft) long tear that formed in the fabric. Once inflated, the balloon stood 96 m (316 ft) tall. The gondola was kept anchored to the earth by a team of more than 100 soldiers holding cables. Preparations were complete by 7:01 am the following morning and the conditions were deemed suitable for a launch.

The crew of the Explorer II consisted of Captain Albert W. Stevens, in command of the mission, and Captain Orvil A. Anderson. A crowd of around 20,000 viewers gathered to watch the event. (The local residents had raised and contributed $13,000 for the mission.) Lift-off occurred at precisely 8:00 am with the release of 34 kg (75 lb) of ballast made of fine lead shot. A few moments after liftoff, wind shear propelled the balloon into a side canyon, but thereafter it ascended normally.