The bishops are believed to have planned to travel to a neutral country, most likely Portugal, where they would elect a new Pope, reports The Telegraph.
It is known that Hitler considered kidnapping the Pope, but Pius XII's considerations only came to light when records were found in the secret Vatican archives by Peter Gumpel, a Jesuit priest responsible for researching whether the wartime pope should be made a saint.
According to Gumpel, “Pius said 'if they want to arrest me they will have to drag me from the Vatican' … the person who would leave the [Vatican] under these conditions would not be Pius XII but Eugenio Pacelli [the Pope’s name before his election]."
Gumpel said, “It would have been disastrous if the Church had been left without an authoritative leader".
"Pius wouldn't leave voluntarily. He had been invited repeatedly to go to Portugal or Spain or the United States but he felt he could not leave his diocese under these severe and tragic circumstances."
On 6 September 1943, just after Italy signed an armistice with the Allies, German forces occupied Rome.
SS General Karl Otto Wolff was ordered to "occupy as soon as possible the Vatican, secure the archives and art treasures and transfer the Pope, together with the Curia so that they cannot fall into the hands of the Allies and exert a political influence."
The plan was never carried out, however, as, General Wolff believed the action would alienate Catholics across the world, according to some historians.
Pope Pius XII has been seen as a controversial figure, with some historians claiming that he was anti-semitic and was supportive of the Nazi regime. However, as more documents and evidence have come to light, the role of the Pope and of the Church in saving Jews from the Nazis has become more recognised.
Some historians argue that the reason Hitler wanted to capture the Pope was because he was concerned Pius XII would continue speaking against the way the Nazis treated the Jews.