Bread: One is not obligated to eat specifically bread on Chol Hamoed, and it suffices even if he eats mere fruits.[1] Nevertheless, initially it is a Mitzvah for one to have a meal with bread on Chol Hamoed[2], [once by day and once by night, as stated above].

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[1] Admur 188:10 and 639:17; M”A 188:7; M”B 530:1

It is for this reason that if one forgot Yaleh Veyavo in Birchas Hamazon of Chol Hamoed that he is not required to repeat Birchas Hamazon. [Admur ibid]

Other opinions: Some Poskim rule that one is obligated to eat two meals on Chol Hamoed which consist of bread, and for this reason if one forget Yaaleh Veyavo he must therefore repeat Birchas Hamazon. [Tashbatz 214 brought in Beis Yosef 188; Nimukeiy Orach Chaim 530:1]

[2] M”A 530:1; Elya Raba; P”M 530 A”A 1; M”B 530:1; Kaf Hachaim 530:4; Nimukeiy Orach Chaim 530:1; See other opinions above that hold eating bread on Chol Hamoed is an obligation

The reason: As it is a Mitzvah to honor Chol Hamoed with food and drink, and the main part of a meal is bread. [M”B ibid] And according to some Poskim [stated above] eating bread on Chol Hamoed is an obligation, and hence although regarding Yaaleh Veyavo we rule Safek Brachos Lihakel, nevertheless regarding the Biblical command of “Mikra Kodesh” one should be stringent and eat bread. [Nimukeiy Orach Chaim ibid]