Flex items with base width

flex-base-width.css

.parent-of-blocks {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
}

.item-as-block {
    flex: 1 1 200px; /* Stretching box */
    flex: 0 1 200px; /* No stretching box */
    margin: 15px;
}