An international design competition was held among 'invited' architectural firms. There were no open competitions or tenders in Malaysia then or even now. There were eight entries for the design competition. They not only had to meet the PM's criteria but also had to be functional and practical.
Eventually the competition was won by the Argentinian-American architect, Cesar Pelli, who also designed the World Financial Center in New York, and the Canary Wharf in London. He presented two slim towers which gradually decreasing circumference pointing upwards, which gives an impression of two elegant towers spiraling towards the heavens.
The important part of the towers is the skybridge. It not only serve the purpose of faciliting human movement across the two towers, it was also symbolic of a sky portal and a gateway to Vision 2020. Vision 2020 was Dr. Mahathir's vision for Malaysia becoming a developed nation by the year 2020.
Initially the Petronas Towers were not designed to be the tallest building in the world. In the original design, it was to be 427 metres. The tallest building then was the 443 metres tall Sears Towers of Chicago. The project was already underway when the Dr. Mahathir suddenly requested that it attempts to be the tallest. After much recalculation, it was declared possible. Not by increasing the number of floors but by extending the height of the pinnacle.
Now you know a little more about Petronas Towers History.