Do I own the IP created by a contractor?

Hi
The US patent attorneys on this portal will be able to give more detailed advice but to answer generally - No, you don't automatically own the IP.
I don't know where your third-party contractors will be based but in many territories, a distinction is made between employees and non-employees. IP created by employees in the “course of business” usually belongs to the employer. For anyone who is not an employee, you need a contract in place not only assigning (transferring) IP to you but describing exactly what is being transferred. This is because the developer may want to license rather than transfer certain elements of IP to you. For instance, a developer working with other clients as part of their own business may need to retain the source code they already use because transferring exclusive use to you would in effect put them out of business once they move on from your project.
You say you hope to turn this into a successful venture, I would strongly advise you don’t scrimp on your legal advice, try to find and negotiate a fee with a good attorney who works in this particular field.