When Montag gets home after first meeting Clarisse, he discovers that she has overdosed on sleeping pills. She is barely breathing; just a faint thread of life is keeping her hanging on. As he walks towards her, he kicks the empty pill bottle with his foot and realizes what she has done, and it is as that moment that the jet bombers flew over the house, rattling everything and hurting his ears. Bradbury probably had that happen at that exact time as a sort of connection--they live in a society that breeds unhappiness and war. Just as Mildred keeps trying to kill herself, war is also constantly on the horizon. Mildred hasn't succeeded as of yet, just like war hasn't been fully declared yet. But, the suicide attempt, and the jet bombers flying overhead are both foreshadowing of the events to come. People are so unhappy that they eventually will succeed in ending their lives, either through suicide or war. The jet bombers going off are also just a great literary tool to use as Montag discovers his wife's barely alive body; it is earth-shattering, it is huge, and it is that moment that he finally realizes how miserable he and his wife really are.