According to this argument...

According to this argument, Abu Bakr became the head of the state of Medina because once upon a time he traveled with Muhammad from one city to another – a truly remarkable exercise in “scientific logic.” In Makkah, the Prophet had made Abu Bakr the “brother” of Umar bin al-Khattab; in Medina, he made him the “brother” of Kharja bin Zayd. At the siege of Khyber, Abu Bakr was given the banner, and he led troops to capture the fortress but without success.

In the campaign of Dhat es-Salasil, Muhammad Mustafa sent Abu Bakr with 200 other ranks under the command of Abu Obaida bin al-Jarrah to reinforce the troops of Amr bin Aas. The latter took command of all the troops. Abu Bakr, therefore, served two masters in the same campaign – first Abu Obaida and then Amr bin Aas. There were many battles and campaigns of Islam but there is no evidence that Abu Bakr ever distinguished himself in any of them.

In the Syrian campaign, the Apostle of God placed Abu Bakr under the command of Usama bin Zayd bin Haritha. The Apostle never appointed Abu Bakr to any position of authority and responsibility, civil or military. Once he sent him to Makkah as the leader of a group of pilgrims to conduct the rites of Hajj (pilgrimage).

But after Abu Bakr's departure, the Apostle sent Ali ibn Abi Talib to promulgate, in Makkah, the ninth chapter of Al-Qur’an al-Majid (Surah Bara'ah or Immunity), the newly revealed message from Heaven. Abu Bakr was not allowed to promulgate it. Ali promulgated it. The only other distinction of Abu Bakr was that just before the death of the Apostle, he led the public prayers.

Montgomery Watt From 622 to 632 he (Abu Bakr) was Mohammed's chief adviser, but had no prominent public functions except that he conducted the pilgrimage to Mecca in 631, and led the public prayers in Medina during Mohammed's last illness. (Encyclopedia Britannia, Vol. I, page 54, 1973) Some writers have claimed that Abu Bakr belonged to the “first Muslim family.” Probably, it means that all members of his family accepted Islam before all members of any other family did.

But if the son and the father of a man are members of his family, then this claim cannot but be false. Abu Bakr's son, Abdur Rahman, fought against the Prophet of Islam in the battle of Badr. It is said that when he challenged the Muslims, Abu Bakr himself wanted to engage him in a duel but was not allowed to do so by the Prophet. Abu Bakr's father, Abu Qahafa, lived in Makkah.