The driver and at least five other men on the bus were drunk and looking for a "joyride," police said.
The men, from a poverty-ridden slum on the outskirts of Delhi, dragged the woman to the back of the bus and beat up her male friend.
Police say the men took turns raping the woman, using an iron rod to violate her as the bus drove around the city for almost an hour. When they had finished, they dumped the two victims by the side of the road.
The woman's injuries were so severe that some internal organs had to be removed. She died two weeks later at a hospital in Singapore.
A rape every 22 minutes
As in many countries, rape is a grimly frequent occurrence in India.
According to Indian government statistics, a woman is raped every 22 minutes on average.
But the New Delhi attack seized the country's attention.
Advocates criticized the world's largest democracy for failing to protect half of its population. Protesters demanded better treatment of women and decried the apathy of police and the judicial system.
The government passed tougher anti-rape laws, introducing the death penalty for repeat offenders, and imprisonment for acid attacks, human trafficking and stalking.
But some Indians say that while the laws on crimes against women have changed, mindsets and enforcement haven't.
Prosecution of such crimes has improved, Bedi believes, but it will take a heavy emphasis on the family and school environments to resolve the problem in the long run.
"You can't just begin and end with the police and the prosecution and the courts," she said. "You have to go backward and take it to the source."