Project Gemini developed spaceflight
technology that would be necessary for the Apollo
Moon flights a few years later. It built on the initial successes of the preceding Mercury
program. Project Gemini is where Americans learned how to fly in space, not just go around in circles.
Since the lunar missions would last up to two weeks, it was necessary to prove that the spacecraft could perform that long, and that the astronauts could withstand the cramped, zero-g environment that long. Up to then, the longest American manned spaceflight had only lasted a day and a half. The long-term physiological effects of spaceflight were a matter of much speculation.
Another big question mark was the practicality of rendezvous and docking in space. The lunar orbit rendezvous mission profile that had been chosen for Apollo required the returning lunar lander spacecraft to catch up to the waiting command module in lunar orbit. Rendezvous had to work with high confidence because there was no possibility of rescue. Rendezvous techniques had been worked out in theory--rather complicated theory, because fuel is precious in space and optimized maneuvers are required--but nobody knew whether the necessary high precision could be achieved in practice.
Gemini was also the first time that American astronauts ventured outside the relative safety of their spacecraft--the first spacewalks or EVAs (extravehicular activities). Apollo surface exploration would be a series of long spacewalks, and some Apollo missions called for spacewalks on the way back from the Moon. Spacewalks would also be necessary for a range of emergency situations in which the astronauts would have to fix something on the outside of the spacecraft.
The original plan called for developing techniques to glide back to a landing on dry ground with a parasail, instead of the ocean landings that had been used in Mercury. This idea was scrapped in 1964 before the first operational flight.
The Gemini spacecraft was just large enough for two astronauts, with very little room to move around, compared to the much larger Apollo spacecraft. Indeed, some of the returning spacewalkers ran into a dangerous situation when they were nearly unable to stuff themselves back into the spacecraft! The prime contractor for the Gemini spacecraft was McDonnell Aircraft in St. Louis.
The Titan II rocket that boosted the Gemini spacecraft into orbit was built by the Martin Company. It was a man-rated version of America's main ICBM at the time.
Many of the Gemini astronauts flew to the Moon in the Apollo program. Two of them, Gus Grisson and Ed White, died in the Apollo launchpad fire during a training exercise in 1967.