How can we optimize AI for the greatest good, instead of profit?

How can we ensure that artificial intelligence provides the greatest benefit to all of humanity? By that, we don't necessarily mean to ask how we create AIs with a sense of justice. That's important, of course--but a lot of time is already spent weighing the ethical quandaries of artificial intelligence. How do we ensure that systems trained on existing data aren't imbued with human ideological biases that discriminate against users? Can we trust AI doctors to correctly identify health problems in medical scans if they can't explain what they see?