After the problems that developed during the MR-2 mission carrying the chimpanzee Ham, it was apparent that the Redstone needed further development before it could be trusted to carry a human passenger.

Dr. Wernher von Braun added Mercury-Redstone BD (Booster Development) to the launch schedule between the MR-2 and MR-3 missions. This went over the protests of some, including astronaut Alan Shepard, who argued that the problems on MR-2 had been quickly identified and easily fixed. Von Braun was adamant that the Redstone could not be considered man-rated until a completely perfect test flight and that the launch vehicle performance on MR-1A and MR-2 had not met acceptable standards for carrying a human passenger.