Here is a good resource site about Hitler’s campaign to rid Germany of “Useless Eaters” and “Lives Devoid of Value”:
http://www.toolan.com/hitler/index.html
From the above cited source:
The next step, towards the end of 1938 and the beginning of 1939, was publicly tested in Germany after endless discussions and propaganda moves. A letter addressed to Adolf Hitler written by a man called Knauer from Leipzig asked for permission for a doctor to shorten the life of his child who was born blind, seemed to be an idiot and had only parts of its arms and legs. The child itself was at this time at the Children’s Clinic in the University of Leipzig, which was headed by Professor Werner Catel, Professor for Neurology and Psychiatry at the same university.
At that time Catel was already an exponent of euthanasia and has remained one to this day which fact he acknowledged in his book “Border Situations of Life - Contribution to the Problem of a Limited Euthanasia”. it was Catel who made the suggestion to the father, or at least focused his attention in that direction, to write a letter to the Führer. As an answer to this letter, Hitler sent his physician, Professor Karl Brandt, to Leipzig and after consultations with Catel, put the child to sleep.
Several months later, Hitler signed a document authorising Dr. Karl Brandt and Reich-leader Philipp Bouhler to permit euthanasia in special cases. This authorisation was supposedly signed in October 1939, but was backdated to the beginning of September of the same year. The document was really nothing more than an authorisation and formulated in such a way that a doctor who truly felt bound to the Hippocratic Oath could interpret it in such a way that no-one would have to die. The “Führer-order” as it was generally called, had apparently come about after a lively discussion between Dr. Karl Brandt, Dr. Leonardo Conti and Philipp Bouhler and was as follows:
“Reichsleader Bouhler and Dr. Brandt M.D. are charged with the responsibility of enlarging the authority of certain physicians to be designated by name in such a manner that persons who according to human judgement can upon most careful diagnosis of their condition of sickness be accorded a mercy death.
Signed - A. Hitler”.