Every kalpa has 14 manvantaras and each manvantara has an Indra. Name of current Indra is Purandhara.
Anybody knows, previous life of the current Indra Purandara? How exactly he became qualified to become Indra?
The present Indra became king of the three worlds by performing a 100 Ashwamedha Yagnas (horse rituals), as described in this chapter of the Shanti Parva of the Mahabharata:
It is heard, O Partha, that the gods and the Asuras fought against each other. The Asuras were the elder, and the gods the younger brothers. Covetous of prosperity, fierce was the battle fought between them. The fight lasted for two and thirty thousand years. Making the earth one vast expanse of blood, the gods slew the Daityas and gained possession of heaven. Having obtained possession of the earth, a (large) number of Brahmanas, conversant with the Vedas, armed themselves, stupefied with pride, with the Danavas for giving them help in the fight. They were known by the name of Salavrika and numbered eight and eighty thousand. All of them, however, were slain by the gods....
The Horse-sacrifice, that grand rite, has been indicated as an expiation for thee. Make preparations for that sacrifice, O monarch, and thou shalt be freed from thy sins. The divine chastiser of Paka, having vanquished his foes with the assistance of the Maruts, gradually performed a hundred sacrifices and became Satakratu. Freed from sin, possessed of heaven, and having obtained many regions of bliss and great happiness and prosperity, Sakra, surrounded by the Maruts, is shining in beauty, and illuminating all the quarters with his splendour. The lord of Sachi is adored in the heavens by the Apsaras. The Rishis and the other gods all worship him with reverence.
So first the gods, specifically the Adityas, i.e. the sons of Kashyapa and Aditi, conquered Devaloka, and then Indra performed a hundred Ashwamedha Yagnas in order to become worthy of being the king of the three worlds. (That is why Indra is known as Shatakratu, or one who has performed a hundred Yagnas.) Then a grand coronation and Rajasuya Yagna was performed for Indra, as described in this chapter and subsequent chapters of the Aitareya Brahmana of the Rig Veda.
And ever since Indra became king of the three worlds, he's tried to make sure that no one else performs a 100 Ashwamedha Yagnas. That's why he tried to steal the horses from the Ashwamedha Yagnas of Rama's ancestor Sagara and Vishnu's incarnation Prithu, and that's why he sought Vishnu's help when Mahabali completed his hundredth Ashwamedha Yagna (which was the impetus for Vishnu's Vamana incarnation).
As far as I know, Indra has always succeeded in protecting his throne from performers of Ashwamedha Yagnas. He has lost his throne militarily on a number of occasions, however; he lost it to Vishnu's incarnation Mandhata as I discuss here, to Vishnu's devotee Prahlada as I discuss here, and to the demons Shumbha and Nishumbha as I discuss here. And he once abdicated the throne which led to Yayati's father Nahusha taking over Yet he always manages to get out of these situations, perhaps because of his devotion to Vishnu; the Vishnu Sahasranamam calls Vishnu "Indrakarma", or one who does great deeds for Indra.