Example code to populate a CMB select field with posts from a custom post type.

metaboxes.php

<?php
/**
 * Get a list of downloads
 * 
 * Return an array of downloads suitable for CMB's select field.
 * 
 * Example CMB select field usage:
 * 
 * array(
 * 	'name' => 'Download',
 * 	'id' => $prefix . 'download',
 * 	'type' => 'select',
 * 	'options' => pw_get_downloads()
 * ),
 * 
 * @author Phil Wylie
 * @link http://goo.gl/c0bPb
 * @return array $downloads_array Array containing titles and post IDs
 */
function pw_get_downloads() {
	$args = array(
		'posts_per_page' => -1,
		'post_type' => 'download',
		'orderby' => 'title',
		'order' => 'ASC'
	);
	$downloads = new WP_Query( $args );
	
	$downloads_array = array();
	
	if ( $downloads->have_posts() ) {
		while ( $downloads->have_posts() ) {
			$downloads->the_post();
			
			$downloads_array[] = array(
				'name' => get_the_title(),
				'value' => get_the_ID()
			);
		}
	}
	
	wp_reset_postdata();
	
	return $downloads_array;
}