If you are a tourist to the United States who is using a visa then it is possible to be able to stay for a longer period of time than that initially granted. However, in order to make certain that you will have more time in the US, you need to first make certain that you will qualify.
Getting a b2 US visa does not, according to the State Department, guarantee being given access into the United States. It does enable you to travel to a United States port of entry and request permission to then enter the country and the decision will then be made by officials from the US Customs and Border Protection and Department of Homeland Security as to whether you have permission to go beyond the seaport or airport.
The officials can deny or permit access in addition to determining the exact length of your stay and the date on which you must depart will be stamped on your I-94 document. This should be kept inside your passport at all times.
It is then possible to put in an application for an extended visit by filling out the Application to Extend/Change Non-Immigrant Status Form I-539. However, this has to be done prior to the date of leaving. Applications made after the expiry date are likely to result in deportation. Applying 45 days in advance is recommended to extend US visas.