It helps to pinpoint where defensiveness comes from. However, a quicker approach is study up on public speaking techniques. Pause, take a sip of water, say great question. That gives the proper amount of time to think. Then answer.

When in business, sometimes questions aren't about doubt, but are about interest in the sales pitch. Maybe they are the middle man or need to clear funds? It's not you personally, it's what they might need to answer/business.