The first version of Kerbal Space Program that has been released to the public is the 0.7.3 version, released June 24, 2011. However, it is not the first version of KSP.
Game development actually began January 17, 2011, when programming of the game began. Early versions were created, and were intended to prove that the project was feasible. However, they contained only the bare minimum of the game's functions. These versions were never made available to the public. There are some images from the blog of Harvester, the head of the team developing the game.
Harvester revealed these images in 2012, to mark the first anniversary of KSP, and gave some information on it. However, the information speaks only of versions 0.0 to 0.4, and does not mention the versions between 0.4 and 0.7.3.
The first version of KSP was created to demonstrate that the potential concept that Harvester had. At that point the proto-KSP was not yet even a game. There were no controls or camera movements, the field was just a green plain, the rocket and the launch pad were gray cubes, and the sky was a default asset (skybox) bundled with Unity engine. This version showed just a rocket taking off automatically, without being able to direct it.
It is in this version that the rocket editor appeared. The VAB did not exist yet, and rockets were built directly on the launchpad. The camera was movable. Interestingly, the game was made in 2.5D: the scenery was in 3D, but the rocket could only be directed to the X and Y axes, not the Z axis. A simple ground had also been created, but it was flat and you could get out, which made the rocket fall in the vacuum.
Kerbin appeared in this version, at least as a sphere of 20 km in diameter in order to develop a realistic gravity. It is also in this version of what became the ancestor of the orbital map, a simple black square indicating the trajectory of the rocket.
In this version, rockets can be staged, and there was a movable camera in flight mode.