For Whom the Bell Tolls is the finest example of modernist novel, as it reflects the typical conventions of modern novel including its style, structure, characterization and themes.
Let us look into its theme for instance; it is existentialist that Hemingway has developed from protagonist Robert Jordan, who finding meaning in his life, caught up in atrocities of war and chaos.
Then look at its style, which author has tightly controlled and formed through realistic and specific details. In doing so, he creates a realistic piece of art that is an essential feature of modern novels.
Finally, let us examine its structure which is also modern. Various characters in the novel tell their stories and thus we see shifts in point of views and the narrative appears like a short stories collection.