Ibrahim and Khalid El Bakraoui initially had carrying out an attack in France, probably during Euro 2016, and another at Amsterdam Airport Schiphol, as their main objectives. This emerges from an investigation by the weekly magazine Paris Match which has been published today (Thursday).
Brussels was not their prime target.
The El Bakraoui brothers probably revised their intended targets to the departure lounge of Brussels Airport and the Maelbeek underground station, as they were under pressure to act urgently. This is thought to have come both after the discovery of a hideout on Rue du Dries, in Brussels on March 15th, and the capture of Salah Abdeslam on March 18th.
This is according to evidence found by investigators, in particular in the computer which the brothers threw away at the bottom of the block of flats on the Rue Max Roos.