In order to apply to a business PhD program in the US, you must have completed at least an undergraduate degree. That degree may be from a variety of fields, not necessarily the one you are applying for. So, an engineer can apply to a marketing program, for example. A graduate degree like a Masters of Science is not required, but it may help to strengthen you application.
In my case, the undergraduate degree is a bachelor's, and the graduate degrees are an Executive MBA and a Master of Science. All of them in the field of business administration. The bachelor's degree and the MBA are responsible for the stage in my life related to acquiring knowledge.
I became a bachelor in business administration at the #1 university in the Latin America. Having a degree from a top school like this may help your application, since the people who review your application will be better able to evaluate your profile when compared with an applicant from an unknown foreign university. My MBA is from a school which was considered one of the top MBAs in Brazil.
I think both were very important to learn about the world of business. But none of them were relevant to learn about research. Since the PhD is a research-oriented program, I think a bachelor's degree and an MBA are a very good start, but far from being enough to prepare me for a PhD.
The Masters of Science was much better in that regard, and I'll write more about it later, since I don't believe the role of a Master of Science program is to acquire knowledge.