NASA laid down guidelines for the development of the two-man spacecraft in a document included as Exhibit "A" in NASA's contract with McDonnell. The development program had five specific objectives: (1) performing Earth-orbital flights lasting up to 14 days, (2) determining the ability of man to function in a space environment during extended missions, (3) demonstrating rendezvous and docking with a target vehicle in Earth orbit as an operational technique, (4) developing simplified countdown procedures and techniques for the rendezvous mission compatible with spacecraft launch vehicle and target vehicle performance, and (5) making controlled land landing the primary recovery mode. The two-man spacecraft would retain the general aerodynamic shape and basic systems concepts of the Mercury spacecraft but would also include several important changes: increased size to accommodate two astronauts; ejection seats instead of the escape tower; an adapter, containing special equipment not needed for reentry and landing, to be left in orbit; housing of most system hardware outside the pressurized compartment for ease of access; modular systems design rather than integrated; spacecraft systems for orbital maneuvering and docking; and a system for controlled land landing. Target date for completing the program was October 1965.

AiResearch Manufacturing Company, a division of the Garrett Corporation, Los Angeles, California, received a $15 million subcontract from McDonnell to manufacture the environmental control system (ECS) for the Gemini spacecraft. This was McDonnell's first purchase order on behalf of the Gemini contract. Patterned after the ECS used in Project Mercury (also built by AiResearch), the Gemini ECS consisted of suit, cabin, and coolant circuits, and an oxygen supply, all designed to be manually controlled whenever possible during all phases of flight. Primary functions of the ECS were controlling suit and cabin atmosphere, controlling suit and equipment temperatures, and providing drinking water for the crew and storage or disposal of waste water.

The Air Force School of Aviation Medicine, Brooks Air Force Base, Texas, began a simulated long-duration Gemini mission. Two men were to live for 14 days in a 100-percent-oxygen atmosphere maintained at a pressure of 5 pounds per square inch, the proposed spacecraft environment.

Manned Spacecraft Center announced specialty areas for the nine new astronauts: trainers and simulators, Neil A. Armstrong; boosters, Frank Borman; cockpit layout and systems integration, Charles Conrad, Jr.; recovery systems, James A. Lovell, Jr.; guidance and navigation, James A. McDivitt; electrical, Sequential, and mission planning, Elliot M. See, Jr.; communications, instrumentation, and range integration, Thomas P. Stafford; flight control systems, Edward H White II; and environmental control systems, personal and survival equipment, John W Young.