No one scientist developed the scientific theory although many helped to develop the method. Aristotle, Galileo Galilei, Roger Bacon and Rene Descartes all contributed to the scientific theory.
Aristotle developed methods to arrive at conclusions using observation and Bacon came up with the cycle of observation, hypothesis, experimentation and verification. Galileo used everything from mathematical deduction to hypotheses and experiments to develop the scientific theory. Descartes wrote the "Discourse on Method" as his contribution to the scientific method. Since then, the scientific method has come to be known as a major way to pursue knowledge by using observation and experimentation and as a major way to test hypotheses.