As per Valmiki Ramayana, two bows were formed by God Devendra, both were of equal capacity. They were given to Rudra and Vishnu to fight with each other to know which one is more powerful. However, just before the war was about to start, Aakashvani said that the war will lead to the total destruction and hence the war was stopped. Later, Rudra threw his Dhanush towards earth, which was later known as “Shiva Dhanush”. It was found by King Devaratha, the ancestor of King Janaka. Another legend story says, Vishwakarma crafted two divine bows. He gave Sharanga to Lord Vishnu and Pinaka (Shiva Dhanush) to Lord Shiva.
Another acknowledgement says, Lord Shiva had gifted his famous bow to Devarata, who was a great devotee of Lord Shiva and he was the ancestor of King Janaka. This bow was extremely powerful and heavy and it was made by Lord Vishwakarma and gifted to Lord Shiva.
It is to be said only Tridev and Tridevi could lift the bow and as Sita was the incarnation of supreme deity Laxmi. In the earlier part of her life, Sita while playing with her sisters had unknowingly lifted the table over which the bow had been placed, which was something no one in the kingdom could do. So King Janaka declared that Sita would be married only to a man who could lift and string the Shiva Dhanush.
The challenge was simple, anyone who lifts and strings the bow, would win Sita’s hand for marriage. There was no swayamvar as per Valmiki Ramayan. Kings and princes were coming one by one to try their luck but none could lift it. Janak wanted her daughter to marry a person who could match her capability and Lord Rama easily lifted and strung the bow as he was the incarnation of Lord Vishnu. Shiva Dhanush had broken due the force took place while stringing the bow, he didn’t mean to break the bow.