The fact that there are definite energy levels in the radiation spectra is known since light wavelength has been linked with the energy quantum (Einstein 1905 - Δ=ℎ.). These energy differences explain the reason why the spectrum of atoms is discontinuous (formed by spectral lines).
Bohr had the idea to link it to a change in the orbit radius of the electron around the nucleus (The idea that H atom could be formed by the association of one proton and one electron had been suggested a couple of years before by E. Rutherford).
This outstanding idea was perfected by a whole series of remarquable physicists in the years 1920–1930. It even continues to be refined to this day, where it forms the basis of atomic spectroscopy.