Gemini began as Mercury Mark II to fill this gap. The concept was to enlarge the Mercury capsule's basic design to accommodate two crew, provide it with orbital manoeuvring capability, use existing boosters to launch it and an existing upper rocket stage as a docking target. The latest aircraft engineering was exploited , resulting in a modularised design that provided easy access to and changeout of equipment mounted external to the crew's pressure vessel.

MOL (Manned Orbiting Laboratory) was the US Air Force's manned space project after Dynasoar was cancelled, until it in turn was cancelled in 1969. The earth orbit station used a helium-oxygen atmosphere, a Gemini capsule accomodating a crew of two for ascent and reentry, and the Gemini retrofire package to return the crew to earth. MOL normally would have remained attached to the Titan III transtage, which would provide it up to 760 m/sec of maneuver capability. Experiments planned ranged the gamut from military reconnaisance using large optical cameras and side-looking radar, through interception and inspection of satellites, to exploring the usefulness of man in space and test of Manned Maneuvering Units. After cancellation, some of the reconnaisance systems ended up in later KH series sattelites, and some of the manned experiements were accomplished on Skylab.

In 1964 the MSC Advanced Spacecraft Technology Division formulated a mission flight plan for using a Gemini spacecraft to link up with an orbiting vehicle to achieve a long-duration space mission (dubbed the "Pecan" mission). The two crewmen were to transfer to the Pecan for the duration of the mission.

A McDonnell concept for using Gemini for extended duration missions. The basic Gemini would dock with an Agena upper stage. Between the docking collar and the Agena is an orbital shelter where the crew would live and work for weeks at a time. The crew would transfer from the Gemini to the miniature space station via an inflatable airlock.

To provide the Gemini spacecraft with a rendezvous and docking target, Agena D rocket stages were modified with the addition of a docking collar, status panel display, and restart capabilities for the Gemini program. After docking, the Agena had sufficient fuel reserves to boost the Gemini into high earth orbits, to the edge of the radiation belts.

The Gemini Ferry vehicle would have been launched by Titan 3M for space station replenishment. A MOL-type hatch in the heat shield would allow the crew to enter the space station through an Apollo-type proble and drogue hatch at the base of the spacecraft - a design very like the Soviet TKS. Much shorter than the MOL, the cargo module could be left attached to the station and the reentry vehicle return to earth on its own.

After docking with the Double Transtage, the first Transtage would be cast off and the second Transtage would propel the Gemini into a circumlunar trajectory. The flights themselves, assuming go-ahead was given in September 1965, would follow immediately after the last Gemini flight. In December 1966 a Titan 3C would drive a 2450 kg circumlunar Gemini capsule to 11 m/s re-entry velocity to verify the heat shield design. This would be followed by a February 1967 manned qualification flight in earth orbit. A manned Gemini would dock with a Double Transtage and be propelled into a high orbit and re-entry speed. In April the sequence would be repeated, this time the Gemini being sent by Transtage into a loop around the moon.

An alternate configuration used two Apollo Service Modules and a repackaged LM descent stage. The first Service Module completed the translunar injection maneuver begun by the S-IVB stage; the second SM accomplished lunar orbit insertion and then functioned as a 'lunar crasher' stage, bringing the Gemini to just above the lunar surface. The Gemini and the third transearth-lunar landing stage would then hover to a landing near the stranded lunar module. The same final stage then boosted the Gemini capsule into a transearth trajectory.

The paraglider was supposed to be used in the original Gemini program but delays in getting the wing to deploy reliably resulted in it not being flown. McDonnell proposed that additional Gemini missions be flown to fully test the paraglider, which was planned for the follow-on Big Gemini.

A version of Gemini was proposed for rescue of crews stranded in Earth orbit. This version, launched by a Titan 3C, used a transtage for maneuvering. The basic Gemini reentry module was extended to 120 inches (3.05 m) diameter to provide a passenger compartment for up to three rescued crew. The same concept would eventually be used for Big Gemini.

The most radical modification of the basic Gemini reentry module ever proposed. Drawing on the results of the ASSET subscale winged reentry vehicle program, McDonnell proposed a version of the spacecraft using the same internal systems but capable of a piloted runway landing. The spacecraft was designed for launch by the standard Titan 2 Gemini Launch Vehicle. Unlike the ballistic Gemini, winged Gemini was not designed to maneuver in orbit (launch on a Titan 3A or 3C with a transtage would be required for that capability). Separation from the launch vehicle was made by 2 x 260 kgf solid motors; attitude control in orbit was provided by 8 x 45 kgf bipropellant thrusters at the base of the vehicle; retrofire was initiated by 5 x 1165 kgf solid rocket motors.

This version of Gemini was studied as a means of rescuing an Apollo CSM crew stranded in lunar orbit. The Gemini would be launched unmanned on a translunar trajectory by a Saturn V. Following lunar orbit insertion it would automatically rendezvous with the disabled Apollo. The three Apollo crew members would transfer by a spacewalk to the passenger compartment of the stretched Gemini reentry module. It would then boost itself and the rescued crew to a transearth trajectory. This version was rejected in favor of the more flexible Gemini Lunar Surface Rescue Vehicle.

Prior to an Apollo moon landing attempt, the shelter would be landed, unmanned, near the landing site of a stranded Apollo Lunar Module. In the event the LM ascent stage would not light to take the crew back to the Apollo CSM in lunar orbit, the two astronauts could go to the shelter and await a rescue mission. The astronaut in the CSM would return alone in the Apollo spacecraft.

Big Gemini ('Big G') was proposed by McDonnell Douglas to the US Air Force and NASA in 1967 as a ballistic manned orbital logistics spacecraft to provide economical resupply of these military and civilian space stations. The capability of existing spacecraft (Apollo CSM, Gemini) for such missions was severely limited. In the 1970-1980 period, it appeared that at least a dozen launches would be required for logistics purposes for the Apollo Applications Program alone. MOL and the AAP workshop would require 3 to 6 flights a year, each flight delivering a crew of two or three, with 1 to 7 tonnes of cargo being sent up and up to 0.6 cubic meters of cargo being returned. Planned late 1970's stations would have crews of 6 to 24, requiring a resupply craft that could deliver up to 12 passengers and 12 tonnes of payload 6 to 14 times a year, returning up to 7 cubic meters of cargo each time. Big G could provide such a capability by 1971, using Gemini technology applied to Gemini and Apollo hardware, with minimum interference to the higher priority Apollo lunar landing program.