731 is the sum of three consecutive primes: 239 + 241 + 251 = 731.
731 is a composite number, being the product of the primes 17 and 43, but 7331, 73331, 733331, and 7333331 are all prime; 731 is the only three-digit composite number with this property.
731 is the largest composite number such that deleting any digit(s) will never yield another composite number. In other words, deletions yield only primes or 1.
Unit 731 was a covert biological and chemical warfare research and development unit of the Imperial Japanese Army.
Source: Prime Curios!