There is an enormous variety of job types within the international development sector and there are roles for people with nearly any area of specialization—be it accounting, psychology or, yes, international development.
Be sure to review some of the interviews with people in this sector. Each one has a different story about how they carved a path in this field.
Almost all of them mention good luck—an opportunity to work in an area they might not have considered, or an opening in a field office that they learned about while overseas, a chance taken that led to new doors being opened. The common threads, though, are persistence, an openness to trying new things, a willingness to do what needs to be done.
Take and create opportunities, be open to new ideas, work hard and don't give up—and good luck will follow.